AcharyaChanakya/simple-wikipedia huggingface dataset:
In particle physics, photons are responsible for electromagnetic force. Electromagnetism is an idea that combines electricity with magnetism. One common way that we experience electromagnetism in our daily lives is light, which is caused by electromagnetism. Electromagnetism is also responsible for charge, which is the reason that you can not push your hand through a table. Since photons are the force-carrying particle of electromagnetism, they are also gauge bosons. Some matter–called dark matter–is not believed to be affected by electromagnetism. This would mean that dark matter does not have a charge, and does not give off light.
A vector is a mathematical object that has a size, called the "magnitude", and a direction. It is often represented by boldface letters (such as formula_1, formula_2, formula_3), or as a line segment from one point to another (as in formula_4).
For example, a vector would be used to show the distance and direction something moved in. When asking for directions, if one says "Walk one kilometer towards the North", that would be a vector, but if they say "Walk one kilometer", without showing a direction, then that would be a scalar.
We usually draw vectors as arrows. The length of the arrow is proportional to the vector's magnitude. The direction in which the arrow points to is the vector's direction.
The Head to Tail method of adding vectors is useful for doing an estimate on paper of the result of adding two vectors. To do it:
It's called the "Head to Tail" method, because each head from the previous vector leads in to the tail of the next one.
Using the component form to add two vectors literally means adding the components of the vectors to create a new vector. For example, let a and b be two two-dimensional vectors. These vectors can be written in terms of their components.
Suppose c is the sum of these two vectors, so that c = a + b. This means that formula_7.
Here is an example of addition of two vectors, using their component forms:
This method works for all vectors, not just two dimensional ones.
The dot product is one method to multiply vectors. It produces a scalar. It uses component form:
The cross product is another method to multiply vectors. Unlike dot product, it produces a vector. Using component form:
Here, formula_13 means the length of formula_14, and formula_15 is the unit vector at right angles to both formula_14 and formula_17.
To multiply a vector by a scalar (a normal number), you multiply the number by each component of the vector:
An example of this is
Soap bubble
A soap bubble is a very thin film of soap water. Their shape is a hollow, round ball, or sphere. They have a colorful surface. Soap bubbles do not last very long. Sometimes they break in the air. When they touch something else, they usually break. When people talk about soap bubbles they think about them being pretty but short-lived. So, they are a symbol for other things that are pretty but short-lived or unimportant. Children enjoy playing with bubbles. Sometimes, adults enjoy seeing art done with soap bubbles.
They get their colors from iridescence.
Fire Brigades Union
A bicycle (or bike) is a small, human powered land vehicle with a seat, two wheels, two pedals, and a metal chain connected to cogs on the pedals and rear wheel. A frame gives the bike strength, and the other parts are attached to the frame. The name comes from these two words - the prefix "bi-" meaning two, and the suffix "-cycle" meaning wheel. It is powered by a person riding on top, who pushes the pedals around with his or her feet.
Riding bicycles, which is also called cycling, is an important way to travel in several parts of the world. The most popular type of cycling is Utility cycling. It is also a common recreation, a good form of low-impact exercise, and a popular sport. Road bicycle racing is the second most popular spectator sport in the world.
Bicycling uses less energy per mile than any other human transport.
In 1817 a German professor, Baron Karl von Drais, created the first two-wheeled bicycle. It was made of wood and had two wheels. The front wheel could be turned using the handlebars in order to steer the bike. However, it did not have pedals, so the rider would have to push their feet on the ground to make it move.
In the 1860s, French inventors added pedals to the front wheel. However, it took a lot of effort to turn the pedals. Later inventors made bikes out of metal only, and made the front wheel very big, giving higher speed. This design was called the penny-farthing bicycle. However, it was difficult to ride, since it could fall easily and the rider would fall far.
Several improvements were made in the 1880s and '90s. In 1885, the safety bicycle was invented. This had two wheels the same size so that the rider could sit at a lower height. It was called the safety bicycle because it much easier to ride than the penny-farthing. When stopping, the rider can simply put down a foot instead of completely dismounting. Instead of pedaling and steering with the front wheel, the safety bicycle steers with the front wheel while the pedals turn the back wheel using a chain. Brakes operated by hand levers on some bikes also increased safety.
In 1888, Scottish inventor John Boyd Dunlop re-invented a type of tire which was filled with air. This made safety bicycles more comfortable. Soon, the freewheel was invented. This was a device inside the hub of the back wheel that allowed the wheel to spin even if the rider wasn't pedaling. However, this meant the rider could no longer stop the bike by backpedaling. As a result, better hand brakes were invented, and a different type of brake which could stop the bike if the pedals were turned backwards. Later inventions included better brakes, and gears which made cycling over hills much easier. During this time the bicycle became very popular.
Basic components common to most bikes include a seat, pedals, gearing, handlebar, wheels, and brakes, all mounted on a frame. The majority also have a gear shifter. The cyclist's feet push the pedals to make them go around in circles, which moves the chain, which turns the back wheel of the bike to make the bike move forwards. The front wheel is connected to the handlebar, so turning the handlebar from side to side swivels the front wheel which steers the bike.
Many thousands of people are hurt every year in bicycle accidents, and hundreds die. Careful riding helps prevent accidents. When riding on streets, it is safest to ride on the same side of the street that cars drive (which would mean riding on the right side of the road in countries where people drive on the right side of the road, and riding on the left in countries where people drive on the left). To avoid hitting people, riders must obey signs that say "no bicycling", even if it does not seem to make sense at the time. Low light makes bicycle lighting important. It may not be safe to ride when it is dark. Riders wear reflective clothing to be safer in low light. Wearing a helmet makes bicycle riding safer. Wearing a helmet does not mean that somebody cannot be hurt if they crash their bicycle, but it makes being hurt less likely. Some bicycles have bells or horns that the rider can use to warn other people that they are riding by them.
Many places have a bicycle path linking houses with shops, schools and stations. These make bicycling safer, letting cyclists stay away from busy motor traffic on dangerous roads.
A wheel is a disc or circle-shaped mechanical device. Its main purpose is to allow things to "roll"; in other words, the wheel spins, and object on the wheels moves more easily along the ground. It is a simple machine.
Most land vehicles roll on wheels. Wheels are often used in pairs, connected by a rod of wood or metal known as an axle. Many machines have wheels with teeth, known as gears. The principle behind the wheel is that of mechanical advantage.
This is a machine in which the wheel and axle turn together. It is the basis of many machines, not just vehicles. The potter's wheel, the lathe and the windlass are examples.
Most experts believe that the ancient Mesopotamians invented the wheel about 4000 BC.
People in Asia also discovered it on their own around 3500 BC. The Inca and Maya had wheels on children's toys around 1500 BC, but they did not use wheels for work. Africa south of the Sahara desert, Europe, and Australia did not have the wheel until people there met people from Europe. Early wheels were solid disks; the spoked wheel was invented around 2000 BC. The earliest documentation of a vehicle with wheels is a painting dated between 33503500 BC.
Physical exercise
The word exercise means:
People do physical exercises as a part of their healthy lifestyle to get stronger and healthier or stay strong and healthy or to improve their body image. There are three basic modes of physical exercises:
Exercises can be an important part of physical therapy, weight loss, or sports performance.
Exercise can be fun, but it is important for the exerciser to do what he likes and eat enough calories , so when he exercises he is building up muscles, not just breaking them down for fuel. Carbohydrates are muscle-sparing; and if one does not get enough calories overall, there is the risk of burning muscles up with the fat to make up the deficit.
Benefits of exercise for all ages have been known since antiquity. Marcus Cicero, around 65 BC, stated: “It is exercise alone that supports the spirits, and keeps the mind in vigor.”
Regular exercise has been shown to help high blood pressure, obesity, heart disease, diabetes, and depression, among others. Exercise also helps people feel less tired.
Percent means "out of one hundred". It is often shown with the symbol "%". It is used even if there are not a hundred items. The number is then scaled so it can be compared to one hundred. Percent is also used to indicate changes in numerical quantities.
For example, we have a bowl of fruit with three apples and one orange. The percentage of apples is 3 out of 4, or 3/4 = 75/100 = 75%.
A percentage is only one way of writing a ratio; one can also write it as a fraction or decimal. There are ways to convert fractions to percentages or decimals to percentages.
Percentages are useful in practice, because it allows one to compare things that are not out of the same number. For example, exam marks are often percentages, which can compare them even if there are more questions on one exam paper than the other.
Writing is the act of recording language on a visual medium using a set of symbols. The symbols must be known to others, so that the text may be read.
A text may also use other visual systems, such as illustrations and decorations. These are not called writing, but may help the message work. Usually, all educated people in a country use the same writing system to record the same language. To be able to read and write is to be literate.
Writing differs from speech because "the readers need not be present at the time". We can read writing from long ago, and from different parts of the world. Text stores and communicates knowledge. Writing is one of the greatest inventions of the human species. It was invented after people settled in towns, and after agriculture started. Writing dates from about 3,300, which is over 5000 years ago, in the Middle East.
Writing today is usually on paper, though there are ways to print on almost any surface. Television and movie screens can also display writing, and so can computer screens. Many writing materials were invented, long before paper. Clay, papyrus, wood, slate and parchment (prepared animal skins) have all been used. The Romans wrote on waxed tablets with a pointed stylus; this was popular for temporary notes and messages. The later invention of paper by the Chinese was a big step forward.
Writing is traditionally done using a hand tool such as a pencil, a pen, or a brush. More and more, however, text is created by input on a computer keyboard.
There are two schools of thought:
Therefore, depending on an author's definition of writing, the term may be used in different ways. With many early systems we do not know what they do convey. The central idea is that "a full writing system must be able to represent anything that might be said in spoken language". From this point of view the invention of the rebus is an essential step.
We can only be sure a sign system is full writing if we can translate the symbols into a modern language. In the case of some ancient scripts we cannot do this.
Writing was invented independently a number of times. The Sumerian, the ancient Egyptian, the Chinese and the Mayan writings are separate in their invention. All these writing systems started with "pictographs", symbols that stood for things. Then they developed a mixture of methods. Our own alphabetic system is different. It is based on the sounds of spoken language. All alphabets are modified versions of the first one, which originated with the Phoenicians and the Ancient Greeks.
The Sumerians lived in Mesopotamia, between the Tigris and Euphrates rivers. 5000 years ago this was a fertile region and is now mostly in Iraq. The Sumerians developed a form of writing called cuneiform. Triangular marks were pressed into soft clay tablets. After the clay had dried in the sun the tablets were baked. Then they were carried somewhere else for others to read. We know that its first uses were for trade, accounting and administration.
The earliest signs were mostly pictorial, but soon they stood as symbols for objects, ideas and sounds. This writing system was extremely successful, and outlasted the Sumerian empire. It was then used by other civilisations in the Middle East, such as the Old
Akkadian, Babylonian, Assyrian, Elamite, Hittite, Old Persian and Ugaritic empires. The last cuneiform inscription was dated as 75. Thus the system had lasted for more than 3000 years. Each version of cuneiform had to be deciphered separately, because all the languages were different. Documents (including stone objects) written in more than one language provided the clues.
This is the most famous of the old forms of writing, or "scripts". It was invented at roughly the same time as cuneiform, yet was quite different in style, and used different materials. Egyptians ended up with three writing systems for the same language. They were:
The writing tools used varied according to the material which was written on. The two cursive scripts were written with reed pens and carbon inks onto papyrus. If the material was cloth, then the writing was done by a brush. Many examples survive. The hieroglyphs were inscribed (carved) into stone (hammer & chisel) or painted onto stone surfaces. Many survive, some with the original colours intact. The key event in the decipherment of hieroglyphic writing was the discovery of the Rosetta Stone. This is a granite slab with the same message written in hieroglyphic, demotic and in Greek. Ancient Greek is well understood, and made possible the interpretation of the other two scripts.
Chinese is the language with the largest number of native speakers. Its history dates back to about 1400. The Chinese writing system is idio-syllabic, a mixed method using characters which may have one or more of these elements:
Chinese has a huge number of characters: in the region of 50,000. Because of this, printing methods were never really successful in China, despite their early invention. In the 14th century Wang Tzhen had sixty thousand wood block characters cut, a huge investment in time and money. He printed 100 copies of a local gazette, and was author of a treatise on agriculture and other technical works. Even with printing machines from Europe in the 19th century, the process was hampered by the huge number of characters, which slowed the "composition" to a snail's pace.
China has eight regional languages that are mutually unintelligible, and many true dialects. The system appears to work mainly because as many as 70% speak Mandarin. Fluency in Chinese reading and writing is undoubtedly difficult to achieve, and this must act as a brake on the drive for literacy. There have been a number of attempts to reform or simplify the system. The most radical in Pinyin, which is a program to replace Chinese characters with an alphabetic system. This was supported by Mao, but faltered after his death.
It seems that the idea of an alphabet–a script based entirely upon sound–arose only once, and has been copied and adapted to suit many different languages. Although no alphabet fits its language perfectly, it is flexible enough to fit any language approximately. It was a unique invention.
Our alphabet is called the Roman alphabet, as compared with the Cyrillic and other alphabets. All of these come from the ancient Greek alphabet, which dates back to about 1100 to 800. The Greek alphabet was probably developed from the Phoenician script, which appeared somewhat earlier, and had some similar letter-shapes.
The Phoenician language was a Semitic language, often called Canaanite. The Semitic group of languages includes Arabic, Maltese, Hebrew and also Aramaic, the language spoken by Jesus. We do not know much about how the alphabetic idea arose, but the Phoenicians, a trading people, came up with letters which were adapted by the early Greeks to produce their alphabet. The one big difference is that the Phoenician script had no pure vowels. Arabic script has vowels which may be shown by diacritics (small marks above or below the line). The oldest Qu'ran manuscripts had no diacritics. Israeli children today use texts with vowel 'dots' added, to about the third grade.
No ancient script, alphabetic or not, had pure vowels before the Greeks. The Greek alphabet even has two vowels for 'e' and two for 'o', to distinguish between the long and short sounds. It is fairly clear from this that careful thought went into both the Phoenician invention and the Greek adaptation. However, no details survive of either process.
Semitic scripts apparently derive from "Proto-Sinaitic", a script of which only 31 inscriptions (plus 17 doubtful) are known. It is thought by some researchers that the original source of this script was the Egyptian hieratic script. By the late Middle Kingdom (about 1900) hieratic had added some alphabetic signs for representing the consonants of foreign names. Egyptian activity in Sinai was at its height at that time. A similar idea had been suggested many years before.
There are a number of scripts which have never been deciphered, despite much effort. Perhaps the most famous are the script of the Indus Valley civilization, and Etruscan. The Indus River civilisation predates other literate civilisations on the Indian subcontinent, going back to about 2500. Their cities of Mohenjo-Daru and Harappa were well-planned, with good drainage. The script is found on seal stones, terracotta, bronze, bone and ivory. All are brief, and the language is unknown.
The Etruscan language used Greek letter-forms, and is found mainly on Etruscan tombs, from Tuscany through to Venice. They were an empire before the Romans, who defeated them, and absorbed their ideas. All knowledge of their language was lost, except that some of the names on tomb memorials can be read from the Greek letters.
The Tărtăria tablets are three tablets discovered in 1961 in the village of Tărtăria (Hungarian: Alsótatárlaka). This is about from Alba Iulia in Romania. The tablets, dated to around 5300 BC, have symbols in clay: the "Vinča symbols". Some claim they are a yet undeciphered language. If this is so, they would be the earliest known form of writing. In 1908 similar symbols were found during excavations, by Miloje Vasić (1869–1956) in Vinča. This is a suburb of Belgrade (Serbia), some 300 km from Turdaș. Later, more were found in another part of Belgrade. Since 1875 over one hundred and fifty Vinča sites have been found in Serbia alone. Many, including Vinča itself, have not been fully excavated. The culture of the whole area is called the Vinča culture. Although some of these symbols look exactly the same as some letters in Etruscan, Greek, and Aramaic, they are generally regarded as an original, independent development.
It is only in the last 150 years that most people have been able to read and write in Europe and North America. In many other parts of the world this did not happen until the 20th century. Until then, literacy was mainly for clerics, that is, people who had training as priests. Even wealthy people were often illiterate, and used scribes to write for them. The invention of printing came before mass literacy. Before 1500, each book had to be created by hand, so there were few books available compared to the billions in the world today. Mass literacy needed cheap books.
Even now, there is still widespread illiteracy.
The ordinary use of writing by means of a pen and paper. Can refer to writing for oneself, as in a diary, but mostly it refers to sending letters. Once it was almost the only means of communication between people who were separated. Now, the telephone and e-mail are the most common means of distance communication.
Rock has several meanings:
Plato was one of the most important classical Greek philosophers. He lived from 427 BC to 348 BC. A wealthy man, he owned at least 50 slaves and created the first university school, called "The Academy". Plato was a student of Socrates (who did not write) and the teacher of Aristotle, who founded another university, known as the Lyceum. Plato wrote about many ideas in philosophy that are still talked about today. He wrote about ideas of deductive reasoning. One modern philosopher, Alfred North Whitehead, said that all philosophy since Plato has just been comments on his works.
Plato wrote his books in the form of dialogues with two people or more talking about ideas, and sometimes disagreeing about them. The "Laws" is Plato's longest dialogue and probably his last.
Socrates is usually the main person in Plato's dialogues. Usually, Socrates talks with people about their ideas, and tries to see if they believe anything that is illogical. Other people in the stories often become angry with Socrates because of this. People who study Plato argue about whether Socrates really said the same things that Plato makes him say, or whether Plato just used Socrates as a character, to make the ideas he was talking about seem more important.
Plato opposed the rhetorics of sophism and insisted on true justice and equality in his work "Gorgias", and on immortality of soul in "Phaedo".
One of Plato's most famous works is "The Republic" (in Greek, "Politeia," or 'city'). In that work, he describes Socrates's vision of an "ideal" state. The method of questioning in this dialogue, called the Socratic method, is as important as the content. "The Republic" contains ideas of Socrates: "Socrates said it, Plato wrote it."
There are many dialogues that were supposed to be written by Plato. This list includes those he probably did write.
A book is a set of printed sheets of paper held together between two covers. The sheets of paper are usually covered with a text, language and illustrations. The book is a more flexible format than the earlier idea of the scroll. The change from scrolls to books began in the Roman Empire, and took many centuries to become complete.
A writer of a book is called an author. Someone who draws pictures in a book is called an illustrator. Books can have more than one author or illustrator.
A book can also be a text in a larger collection of texts. That way a "book" is perhaps written by one author, or it only treats one subject area. Books in this sense can often be understood without knowing the whole collection. Examples are the Bible, the Quran all of them consist of a number of “religious books” in this sense of the word. Encyclopedias often have separate articles written by different people, and published as separate volumes. Each volume is a book.
"Hardcover" books have hard covers made of cardboard covered in cloth or leather and are usually sewn together. "Paperback" books have covers of stiff paper and are usually glued together. The words in books can be read aloud and recorded on tapes or compact discs. These are called "audiobooks".
Books can be borrowed from a library or bought from a bookstore. People can make their own books and fill them with family photos, drawings, or their own writing. Some books are empty inside, like a diary, address book, or photo album. Most of the time, the word "book" means that the pages inside have words printed or written on them.
Some books are written just for children, or for entertainment, while other books are for studying something in school such as math or history. Many books have photographs or drawings.
There are two main kinds of book text: fiction and non-fiction.
These books are novels, about stories that did not happen, and have been imagined by the author. Some books are based on real events from history, but the author has created imaginary characters or dialogue for the events.
Books of non-fiction are about true facts or things that have really happened. Some examples are dictionaries, cookbooks, textbooks for learning in school, or a biography (someone's life story).
Between the written manuscript and the book lie several inventions. Manuscripts are hand-made, but books are now industrial products.
A common type of manuscript was the scroll, which was a long sheet rolled up. The sheet could have been made of papyrus (made by the Egyptians, by weaving the inner stems of the papyrus plant and then hammering them together), or parchment or vellum (very thin animal skin, first used by the ancient Greeks), or paper (made from plant fibers, invented by the Chinese). Manuscripts of this kind lasted to the 16th century and beyond. Turning the manuscript into a book required several developments.
The Romans were the first people to put separate pieces of manuscript between covers, to form a "codex". This was more convenient to handle and store than scrolls, but was not yet a book as we understand it.
Scrolls and codices were written and copied by hand. The Chinese invented "woodblock printing", where shapes are carved out of a block of wood, then ink is applied to the carved side, and the block is pressed onto paper. This woodcut method was slow because the symbols and pictures were made by cutting away the surrounding wood.
Johannes Gutenberg was the first to invent a machine for printing, the printing press, in the 15th century. This involved more than just a press, it involved the production of movable metal type suitable for the machine process.
Initially, the machines were slow, and needed a printer's muscle to make them work. The Industrial Revolution brought steam power and later electrification.
Paper had been invented in China in the 8th century, but it was kept secret for a long time. In Europe hand-made paper was available from about 1450. It was cheaper than parchment but still expensive, and the early printing was a slow process. Therefore, books remained rare. In 1800 the first machines for making paper from wood pulp were invented. New kinds of inks were also invented for various purposes, and machines were driven by steam engines and later by electricity.
The common cheap supply of paper fed the faster printing machines, and books became cheaper. At the same time, in America, Britain and continental Europe, more people learnt to read. So, in the 19th century, many ordinary people could afford to buy books and could actually read them. Also in the 19th century came public libraries, so poorer people could get access to the best books.
Printing was done on large sheets of paper, which were then folded, guillotined (cut) and sewn into the covers. Bookbinding and all the other processes have been done by machines since the 19th century.
Today some of the technologies have been changed, especially those involving illustration and typography. However, books look much the same as they did, with more illustration in colour, but basically the same. That is because experience has shown that readers need certain things for pleasurable reading. Graphic design and typography are the practical arts used to make books attractive and useful to readers.
A sundial shows the current solar time during the day. It does this because the sun appears to move through the sky. At different times in the day when the sun is shining, a shadow is cast in different places on the dial. A person marks the dial with the time at a certain shadow. This lets users easily see the time. There are a few commonly seen designs, such as the 'ordinary' or standard horizontal garden sundial. However, sundials can be designed for any surface where a fixed object casts a predictable shadow. They may be horizontal or vertical or tilted.
Ancient Egypt had sundials. Other cultures developed them further, including the Greeks and Romans.
A peninsula is a region of land that sticks out in a body of water. It is also defined as a piece of land with water on three sides.
Many countries are on peninsulas, and may either take up part of a peninsula (such as Portugal) or all of it (such as India). Parts of a country may also be on a peninsula. For example, Jutland is a peninsula, as is Baja California in Mexico.
If Eurasia is considered a continent then the continent of Europe is technically a peninsula. In this case, the great peninsulas of Europe, the Iberian, Italian, and Balkan peninsulas would technically be peninsulas within peninsulas. The area of the European Peninsula, comprised of Europe minus Russia, Fennoscandia, and any islands, is roughly 4.4 million square kilometers.
The Arabian Peninsula is usually cited as the largest peninsula in the world with an area of 3,237,500 square kilometers.
A derivative is something created from a Primary source, and can mean different things.
Combinatorial game theory
Combinatorial game theory, also known as CGT is a branch of applied mathematics and theoretical computer science that studies combinatorial games, and is distinct from "traditional" or "economic" game theory. CGT arose in relation to the theory of impartial games, the two-player game of Nim in particular, with an emphasis on "solving" certain types of combinatorial games.
A game must meet several conditions to be a combinatorial game. These are:
Combinatorial Game Theory is largely confined to the study of a subset of combinatorial games which are two player, finite, and have a winner and loser (i.e. do not end in draws.)
These combinatorial games can be represented by trees, each vertex of which is the game resulting from a particular move from the game directly below it on the tree. These games can be assigned game values. Finding these game values is of great interests to CG theorists, as is the theoretical concept of game addition. The sum of two games is the game in which each player on her/his turn must move in only one of the two games, leaving the other as it was.
Elwyn Berlekamp, John Conway and Richard Guy are the founders of the theory. They worked together in the 1960s. Their published work was called "Winning Ways for Your Mathematical Plays".
In the theory, there are two players called "left" and "right". A game is something that allows left and right to make moves to "other games". For example, in the game of chess, there is a usual starting setup. One could also, however, think of a chess game after the first move as a different game, with a different setup. So each position is also called a game.
Games have the notation {L|R}. formula_1 are the games the left player can move to. formula_2 are the games the right player can move to. If you know chess notation, then the usual chess setup is the game
The dots "..." mean there are many moves, so not all are shown.
Chess is very complex. It is better to think of easier games. Nim, for example, is much simpler to think about. Nim is played like this:
The easiest game of Nim starts with no counters at all! In such a case, neither player can move. That is shown as , or 0). If instead right moves first, there will be no more moves for left. So both left and right can make a move to 0. That is shown as <nowiki>, or {0|0}</nowiki>. The first player to move will win. Games equal to {0|0} are very important. They are written with the symbol, * (star).
Nim is a simple game used for examples in combinatorial game theory. The rules of nim are simple:
There is a simple mathematical strategy to play the game perfectly. If both players play perfectly, the winner is determined by the initial setup.
Blue is one of the colors of the rainbow that people can see. It is one of the seven colors of the rainbow along with red, orange, yellow, green, indigo and violet. Apart from indigo and violet, it has the shortest wavelength of these colors (about 470 nanometers).
Blue is the color of the Earth's sky and sea. Earth looks blue when seen from outer space by astronauts.
Blue is a color used to show coldness. It is also one of the colors your lips can turn if you are chilly.
Blue is the color of a "bluebird". A blue jay is the color blue.
The color blue is sometimes associated with sadness, which may be where the name of the music style The Blues comes from.
Blue is a color of the Jewish religion.
In Western cultures since the 1940s, blue is associated with men/boys, while pink is associated with women/girls. Before the 1940s, it was the other way around, so blue was for women/girls and pink was for men/boys. Nobody knows exactly why it changed.
Memory is the personal record of past experiences. It is the brain's ability to store information to use later. The Greeks considered memory (Mnemosyne) as the mother of Muses, for without her there would be no art and no science.
Memory is studied in sciences known as "cognitive psychology".
There are two types of memory: short-term and long-term memory, or known as working memory and reference memory.
Murder is when one person kills another person on purpose. If a person does something that makes someone else die, without the intent of killing, it is homicide and may be manslaughter. An accident caused by carelessness may be negligent homicide. Sometimes, a death caused by someone else may not be a crime, for example killing in self-defense.
A person who commits murder is called a murderer. The legal definition of "murder" and "manslaughter" may be different in different countries, and is very much argued on: for example, killing in war is not usually called "murder" by those who fight in the war. Killing in self defense (if people being attacked with death seeming imminent kill the attacker, or someone kills an intruder who has burglarized their dwelling) is usually not "murder", but sometimes manslaughter. Many countries do not even have manslaughter as a legal concept, and only have murder; the countries that do not have manslaughter charge with murder instead. In fact, the United States did not have manslaughter on the books until the 20th century. England was the first country to indoctrinate manslaughter, and the United States was second.
A total of 12,658 murders were reported in 1999 in the U.S., including 1,903 without a specified reason, and 3,779 for which the reasons were unknown.
New York's status as the murder capital of the world improved during the 1990s. What follows are the number of murders for 1999:
Attempted murder is a crime. Attempted murder, or "attempt murder" in common law countries, also sometimes called "murder attempt", is when someone tries to kill another person. Just planning a murder is not enough. The act must come close to, but does not actually take the life of the other person.
It was punishable by death and the execution was carried out in Ancient Egypt, Ancient Mesopotamia, the Indus River Valley, Ancient Greece, The Roman Empire, and of course, China, which is by far the most populated country in the world, to this day. As with murder, attempted murder is a mortal sin to the Catholic Church.
The Roman Empire carried out about ten thousand executions, most if not all beheadings, during its time, and there is no doubt that many of these were those convicted of attempted murder.
"Assassination" is a word which means murdering a prominent person for a reason. The word is most used when the person who was killed was a celebrity or was a person involved in politics. The people who carry out assassinations are called "assassins" or "Hit-men". An assassin may murder someone for political reasons, for money, or for other reasons, such as favors owed and revenge. Assassination is the same as murder and is mortal sin to the Catholic Church.
The word "Assassin" comes from Hashishin, a Muslim group that was active in the Middle East from the 8th to the 14th centuries. This secret society killed people for political and religious reasons. It is thought that the assassins were under the influence of hashish and opium during their killings or during their training. The word "assassin" comes from either "hashashim", the influence of the drugs, or "hassansin", after their leader, Hassan-i Sabbah.
A murdersuicide is when a person kills someone else (murder), and then they kill themselves (suicide).
No one is tracking the number of murder–suicides in the United States. However, medical studies estimate between 1,000 and 1,500 deaths per year in the United States are murder-suicides.
Birds (Aves) are a group of vertebrates which evolved from dinosaurs. Technically speaking, they "are" dinosaurs.
Birds are endothermic, with feathers.
Modern birds are toothless: they have beaked jaws. They lay hard-shelled eggs. They have a high metabolic rate, a four-chambered heart and a strong yet lightweight skeleton.
Birds live all over the world. They range in size from the 5 cm (2 in) bee hummingbird to the 2.70 m (9 ft) ostrich. They are the class of tetrapods with the most living species: about ten thousand. More than half of these are passerines, sometimes known as perching birds.
Birds are the closest living relatives of the Crocodilia. This comes about because they are the two main survivors of a once huge group called the Archosaurs.
Modern birds are "not" descended from "Archaeopteryx". According to DNA evidence, "modern" birds (Neornithes) evolved in the Middle to Upper Cretaceous. More recent estimates, using a new way of calibrating molecular clocks, showed that modern birds originated early in the Upper Cretaceous. However, diversification occurred around the Cretaceous–Palaeogene extinction event.
The Cretaceous–Palaeogene extinction event 66 million years ago killed off all the non-avian dinosaur lines. Birds, especially those in the southern continents, survived this event and then migrated to other parts of the world.
Primitive bird-like dinosaurs are in the broader group Avialae. They have been found back to the mid-Jurassic period, around 170 million years ago. Many of these early "stem-birds", such as "Anchiornis", were not yet capable of fully powered flight. Many had primitive characteristics like teeth in their jaws and long bony tails.
Birds have wings which are more or less developed depending on the species. The only known groups without wings are the extinct moa and elephant birds. Wings, which evolved from forelimbs, gave birds the ability to fly. Later many groups evolved with reduced wings, such as ratites, penguins and many island species of birds. The digestive and respiratory systems of birds are also adapted for flight. Some bird species in aquatic environments, particularly seabirds and some waterbirds, have evolved as good swimmers.
Some birds, especially crows and parrots, are among the most intelligent animals. Several bird species make and use tools. Many social species pass on knowledge across generations, a form of culture. Many species annually migrate great distances. Birds are social. They communicate with visual signals, calls and bird songs. They have social behaviours such as cooperative breeding and hunting, flocking and mobbing of predators.
Most bird species are socially monogamous, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species are polygynous (one male with many females) or, rarely, polyandrous (one female with many males). Birds produce offspring by laying eggs which are fertilised by sexual reproduction. They are often laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching. Some birds, such as hens, lay eggs even when not fertilised, though unfertilised eggs do not produce offspring.
Many species of birds are eaten by humans. Domesticated and undomesticated birds (poultry and game) are sources of eggs, meat, and feathers. Songbirds, parrots and other species are popular as pets. Guano is harvested for use as a fertiliser. Birds figure throughout human culture. About 120–130 species have become extinct due to human activity since the 17th century and hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational bird-watching is an important part of the ecotourism industry.
Birds come in a huge range of colours. These colours can be useful to a bird in two ways. Camouflage colours help to hide the bird, and bright colours identify the bird to others of the same species. Often the male is brightly coloured while the female is camouflaged.
Many birds are brown, green or grey. These colours make a bird harder to see: they camouflage the bird. Brown is the most common colour. Brown birds include: sparrows, emus, thrushes, larks, eagles and falcons and the female birds of many species such as: wrens, ducks, blackbirds and peafowls. When a brown bird is in long grass or among tree trunks or rocks, it is "camouflaged". Birds that live in long grass often have brown feathers streaked with black which looks like shadows. A bittern is almost invisible in long reeds because its camouflage is helped by its posture (beak and head pointed upwards). Other birds, including starlings and minahs, are quite dark in colour, but are flecked with little spots that look like raindrops on leaves. Bird may also camouflage their nests.
Many birds from hot countries are green or have some green feathers, particularly parrots. Birds that live in green trees often have green backs, even if they have bright-coloured breasts. From the back, the birds are camouflaged. This is very useful when sitting on a nest. The bird's bright-coloured breast is hidden. Budgerigars are bred in different colours such as blue, white and mauve, but in the wild, they are nearly all green and yellow. Even though they fly very well, they normally spend a lot of time on the ground, eating grass seeds. Their yellow and black striped back helps to hide them in the shadows made by long dry grass, while their green breasts are a similar colour to the leaves of gum trees.
Grey birds include most pigeons and doves, cranes, storks and herons. Grey birds are often rock-living birds like pigeons or birds that sit on dead tree trunks looking like a broken branch. Water birds like herons often have a pale grey colour which makes it harder for a fish to notice that the bird is standing, looking down for something to catch. Water birds, no matter what colour they are on top, are often white underneath, so that when a fish looks up, the bird looks like part of the sky.
Black birds include crows, ravens and male blackbirds. Some birds that are dark colours spend quite a lot of time on the ground, hopping around in the shadows under bushes. Among these birds are the male blackbird and the satin bowerbird which is not black but very dark blue. Crows and ravens often perch high on bare trees in the winter, where their black shape against the sky looks like the dark bare branches.
Many birds are not camouflaged, but stand out with vivid colours. They are usually male birds whose females are dull and camouflaged. The function of the colours is two-fold. First, the colours help them get mates, and second, the colours identify them to other males of the same species. Many birds are territorial, especially in the nesting season. They give out territory sounds and are easily seen. This lets other males know they will defend their territory. It sends out a "look elsewhere" signal to their competitors.
Some birds are famous for their colour and are named for it, such as the bluebird, the azure kingfisher, the golden pheasant, the scarlet macaw, the violet wren and the robin.
Many other birds are very brightly coloured, in countless combinations. Some of the most colourful birds are quite common, like pheasants, peacocks, domestic fowl and parrots. Colourful small birds include blue tits, the gold finches, humming birds, fairy wrens and bee eaters (which are also called rainbow birds). Some birds, like those of the bird of paradise in Papua New Guinea have such beautiful feathers that they have been hunted for them.
The peafowl is the best example of a display of colour to attract a mate. Also the male domestic fowl and junglefowl have long shiny feathers above his tail and also long neck feathers that may be a different colour to his wings and body. There are only a very few types of birds (like the eclectus parrot) where the female is more colourful than the male.
<nowiki>"Pied birds"</nowiki> are black and white. Black and white birds include magpies, pied geese, pelicans and Australian magpies (which are not really magpies at all). Pied birds often have brightly coloured beaks and legs of yellow or red. The silver pheasant, with its long white tail striped with fine bars of black, has a brightly coloured face.
Most birds can fly. They do this by pushing through the air with their wings. The curved surfaces of the wings cause air currents (wind) which lift the bird. Flapping keeps the air current moving to create lift and also moves the bird forward.
Some birds can glide on air currents without flapping. Many birds use this method when they are about to land. Some birds can also hover and remain in one place. This method is used by birds of prey such as falcons that are looking for something to eat. Seagulls are also good at hovering, particularly if there is a strong breeze. The most expert hovering birds are tiny hummingbirds which can beat their wings both backwards and forwards and can stay quite still in the air while they dip their long beaks into flowers to feed on the sweet nectar.
Different types of birds have different needs. Their wings are adapted to suit the way they fly.
Large birds of prey, such as eagles, that spend a lot of time soaring on the wind have wings that are large and broad. The main flight feathers are long and wide. They help the eagle to stay on rising air currents without using much energy, while the eagle looks at the ground below, to find the next meal. When the eagle sees some small creature move, it can close its wings and fall from the sky like a missile, opening its great wings again to slow down as it comes to land. The world's largest eagle, the Philippine eagle has a wingspan of about 2 m (6.7 ft) wide.
Birds that live in grassland areas or open forests and feed on fruit, insects and reptiles often spend a lot of time flying short journeys looking for food and water. They have wings that are shaped in a similar way to eagles, but rounder and not as good for soaring. These include many Australian birds like cockatoos.
Birds such as geese that migrate from one country to another fly very long distances. Their wings are big and strong, because the birds are large and they stock up on food for the long flight. Migrating water birds usually form family groups of 12-30 birds. They fly very high, making use of long streams of air that blow from north to south in different seasons. They are very well organised, often flying in a V pattern. The geese at the back do not have to flap so hard; they are pulled on by the wind of the ones at the front. Every so often, they change the leader so that the front bird, who does most work and sets the pace, can have a rest. Geese and swans are the highest-flying birds, reaching 8,000 metres or more when on migration. Geese often "honk" loudly while they are flying. It is thought that they do this to support the leader and help the young ones.
Birds that fly very quickly, such as swifts and swallows, have long narrow pointed wings. These birds need great speed because they eat insects, catching most of them while they are flying. These birds also migrate. They often collect in huge flocks of thousands of birds that move together like a whirling cloud.