There are a number of tiny tree frogs which develop from eggs laid in water pools on tropical leaves many meters above the ground. Most of these species do not come to the ground except to mate. Tree frogs have evolved a number of times, but they all have almost identical adaptations. Many millions of years of convergent evolution have resulted in almost identical morphology and habits.
Overall, frogs are a successful and widespread group. There are about 4,800 recorded species, which is over 85% of living amphibian species. They are one of the five most diverse vertebrate orders.
Chemical compound
A chemical compound is a chemical formed from atoms of different chemical elements. The different atoms are joined by chemical bonds. Each compound has a fixed ratio between the elements.
The elements stick together so strongly that the compound behaves like one substance. The chemical that is made depends upon what atoms it is made of and how they are joined together.
Chemical compounds can be liquids, like water which is made from atoms of hydrogen and oxygen joining together. They can also be solids, like sodium chloride that contains sodium and chlorine atoms. We get compounds from natural sources such as plants or make them in laboratories. Compounds can be made from others by scientists and these are called "synthetic chemicals".
When different compounds are mixed together they may have a chemical reaction.
Some chemical compounds can be dangerous but are safe if they are used the right way. Most dangerous chemicals are only used by scientists who have had special training in how to handle and use them. These chemicals might be poisonous, corrosive, explosive, they may catch fire easily, or they may react with other substances.
People who work in factories near chemicals often wear special clothes that stop the chemicals from hurting their body.
Wood
Wood is the main substance in trees. It is mainly formed by the xylem vessels which carry water up the plant. The two main substances in wood are cellulose and lignin. Wood is used to make buildings and furniture, and also for art. Firewood is a fuel. Paper is made from wood fibres. Wood is a renewable resource although it has become scarcer in recent centuries.
Wood is hard to cut, but it is also strong. A lumberjack is a person who cuts down trees. After a tree falls, the wood in it can be cut into long, straight pieces called lumber. Lumber can then be used to make posts, or put together with nails, screws, or even glue to make wooden frames for other shapes.
Wood comes in many different kinds. Oak, maple (hardwood) and pine and redwood (softwood) are widely used types of wood. Woods are usually divided into softwood (from conifers) and hardwood from flowering plants.
For a long time and even today, many buildings, mostly houses, have been made of wood. To build a house with wood, lumber is put together into frames that are the shape of each wall, floor, and roof of the house. Then the frames are placed into the shape of a house. Then the frames can be covered to make solid walls. Sometimes the walls are made of more wood.
When the outside of a house or building is covered in wood, the wooden pieces are usually flat and stacked. These pieces are called shingles. Wood is also sometimes used in other parts of the house, like doors and staircases. Wood is also used to make fences.
Carpenters make houses of mostly soft wood such as pine. For many kinds of furniture they use harder wood such as maple or oak. When someone builds something with wood, they often paint it. Paint protects and beautifies the wood. Some people like the look of wood, so they put clear paint called "varnish" on it. This helps to protect the wood and gives it a shiny finish.
Some people make art with wood. Sometimes sculptures are built with wood: see Grinling Gibbons.
Regular pencils are made of wood. Inside is the "lead", which is not actually lead. Clay or wax and graphite form the "lead" in a pencil.
Wood is turned into paper in large factories called paper mills. Heat, chemicals and machines separate the cellulose fibers from other parts and press the fibers into paper.
Chalk
Chalk is a type of limestone.
Air and water do not wear chalk away easily, so when chalk is next to the sea, it often makes a large cliff. The "White Cliffs of Dover" in Kent, England are a good example of this. When chalk is near the top of the ground, it often makes chalk hills.
Chalk holds water, so these chalk hills may have a lot of water in them. When the weather is very dry, water comes slowly from the chalk. Chalk is mostly formed from the calcareous (CaCO) skeletons of countless tiny planktonic algae called coccoliths. It was laid down in the Upper Cretaceous period.
Blackboard chalk is not real chalk. It is really gypsum (calcium sulfate), but people call it "chalk". People use it for drawing on hard things, because it is soft. If this chalk is rubbed on something hard or rough, it will leave a mark. People often write with chalk on a chalkboard or blackboard. A chalk eraser or water can be used to clean the chalk marks away so the board is clean again. Shops usually sell blackboard chalk in sticks that are about 5 cm long.
There are also big pieces of soft chalk for children to write on the ground with, for example on concrete (such as a sidewalk) or on asphalt (such as a driveway). It is usually okay to write on the ground with this kind of chalk, because the rain will wash the chalk away.
Tailors' chalk is not real chalk either. It is talc (magnesium silicate). Tailors use it to draw on material when they are making clothes.
Utilitarianism
Utilitarianism is the view that the right thing to do is whatever is most useful. It is a about ethics. The word "utilitarianism" comes from the word "utility", which means "usefulness". In most forms of utilitarianism, things that increase human well-being or happiness are called useful. This is not limited to the happiness caused by a single action but also includes the happiness of all people involved and all future consequences.
In general, the theory of utilitarianism is that whatever brings the most happiness to the greatest number of people is the right thing to do.
The theory was made popular by 18th and 19th century British philosophers like Francis Hutcheson, Jeremy Bentham, and John Stuart Mill, but the idea goes all the way back to ancient times.
Bentham wrote about this idea with the words "The greatest good for the greatest number," but did not use the word "utilitarianism". It was Mill, a follower of Bentham's ideas, who named the idea.
Many philosophers argue that utilitarianism has important practical implications for how we should live ethically. For instance, according to utilitarianism we should help the poor, prevent animal suffering, and ensure that future generations have good lives.
Maslowism or Maslow's hierarchy of needs can be divided into social and physical needs. Social needs in Maslowism include physiological, safety, love, esteem, and self-actualization. Maslowist themes like physiology suggests you need food while Maslowist themes like love suggests humans need sex.
Village
A village is a place where people live, normally in the countryside. It is usually larger than a hamlet and smaller than a town or city. In some places, it may be a kind of local government. The dwellings in a village are clustered fairly close to one another, not scattered broadly over the landscape.
In the past, villages were where most people lived. Villages were a usual form of community for societies that do subsistence agriculture, and for some non-agricultural societies. After the industrial revolution, when people started making a lot of things in factories, people lived more in towns. Moving to towns is called urbanization. When villages grow a lot they can grow into towns and then cities. This is what happened to Dubai and Chicago.