There are thousands of symbols that are recognized by most people all over the world, and many more that are limited to certain regions, religions, sciences, etc.. Some of the best known ones are listed below.
September
September (Sep.) is the ninth month of the year in the Gregorian calendar, coming between August and October. It has 30 days. Its name comes from the Latin word "sept" for "seven" (it was the seventh month of the year, before January and February were added to the beginning of the year.)
September always begins on the same day of the week as December, but never ends on the same day of the week as any other month.
In the old Roman calendar, September was the seventh month, which is where it got its name ("Septem" means "seventh"). The ninth month at the time was November ("Novem" means "ninth"). With Julius Caesar's calendar reform, September became the ninth month, with 30 days. September comes after August and before October.
September begins on the same day of week as December every year, as each other's first days are 13 weeks (91 days) apart. No other month of any year, common or leap year, ends on the same day of the week as September.
In common years, September starts on the same day of the week as April and July of the previous year, and in leap years, October of the previous year. In common years, September finishes on the same day of the week as April and December of the previous year, and in leap years, July of the previous year. In leap years and years immediately after that, September starts on the same day of the week as January of the previous year.
In years immediately before common years, September starts on the same day of the week as June of the following year, and in years immediately before leap years, March and November of the following year. In years immediately before common years, September finishes on the same day of the week as March and June of the following year, and in years immediately before leap years, August and November of the following year.
September is one of two months to have an equinox (the other is March, its seasonal equivalent in both hemispheres), where both day and night are roughly of equal length, occurring either on the 22nd or 23rd, halfway between the June and December solstices. In the Northern Hemisphere, Autumn (Fall) begins in this month, while in the Southern Hemisphere, this is the beginning of Spring. For meteorologists, the people who study the weather, these seasons begin on September 1 in those hemispheres. In most Northern Hemisphere countries, school starts in this month, following the summer holidays.
In Ancient Greece, September was called "Boedromion". The Anglo-Saxons called it "Gerstmonath", meaning "Barley month", referring to the harvest. In other countries, it is referred to as "Autumn Month", such as in Finland (Syyskuu) and German-speaking parts of Switzerland (Herbstmonat).
Ethiopian New Year occurs in September. Jewish New Year also often occurs in this month.
Sound
Sound is caused by sound waves. It can be heard when goes through a medium to the ear. All sounds are made by vibrations of molecules. For example, when a person hits a drum or a cymbal the object vibrates. These vibrations make air molecules move. Sound waves move away from where they came from. When the vibrating air molecules reach our ears, the eardrum vibrates, too. The bones of the ear vibrate in way the object that started the sound wave vibrated.
There are three different mediums. They are solids, liquids and gas. Sound travels the fastest through solids because the air particles are closer together than gas and liquids.
These vibrations let you hear different things. Even music is vibrations. Irregular vibrations are noise. People can make very complex sounds. We use them for speech.
Sound waves are longitudinal waves with two parts: "compression" and "rarefaction". Compression is the part of the sound waves where the molecules of air are pushed ("compressed") together. Rarefaction is the part of the waves where the molecules are far away from each other. Sound waves are a sequence of compression and rarefaction.
There are some things that sound can't travel through like vacuum. Vacuum is when there is no air. So if a sound made it would not be able to travel. A vacuum is a space that has no matter. The word comes from the Latin adjective "vacuus" for "vacant" or "void".
Sound waves can travel through solids, liquids, and gases. The thicker the medium, the faster it goes, but they cannot travel through a vacuum (a place with nothing in it). This is why astronauts cannot talk to each other in space: they need a radio to hear each other. Sound can travel through water faster than through air; and even faster in solids like stone, iron, and steel. Sound travels at 335 metres (1,100 feet) per second in the air.
"Pitch" is the highness or lowness of sound. Pitch is how humans hear different frequencies. Frequency is determined by the number of vibrations per second. The highest key on a piano, for instance, vibrates 4,000 times per second. It has a frequency of 4000 hertz (Hz), or 4 kilohertz (kHz). Lower keys have lower frequencies. A note an octave higher than another note has twice the frequency of that note.
The intensity of a sound is how much sound energy goes through a square meter in one second. Sound waves with higher amplitude (bigger vibration) have higher intensity. The intensity of a sound is higher closer to the sound source. Farther away, it's less intense. The inverse-square law shows how sound intensity becomes smaller, farther from the source. "Inverse square" says that when distance gets multiplied by a number, sound intensity gets divided by that number squared (the number times itself). Thus, twice the distance means a quarter the intensity.
Sound intensities can be very different. They can range from 0.000000000001, which are barely heard, to 1 W/m (painfully loud). The decibel scale makes sound intensity numbers easier to work with. A 0.000000000001 W/m intensity is 0 dB (decibels). It is an exponential scale, so when the decibel number increases by 10, the intensity is ten times as much. So, a 1 W/m intensity is 120 dB.
Loudness is how people sense the intensity of sound. Loudness depends on sound intensity, sound frequency, and the person's hearing.
Audible sound has frequencies between 20 Hz to 20 kHz. Human beings can hear audible sound. Sound waves with frequency above 20 kHz are called ultrasound waves. Sound waves with frequency below 20 Hz are called infrasound waves. Human beings cannot hear ultrasound waves and infrasound waves, but some animals, like bats and dolphins, use them. Older people have an even smaller hearing range. People are best at hearing sounds between 1000 Hz and 6000 Hz.
When a sound source is moving towards someone, the frequency seems to increase. The same thing happens when someone moves toward the sound source. Frequency seems to decrease when someone moves away from a sound source. It also seems to decrease when the sound source moves away from someone. This is called the Doppler effect.
Halpern, Alvin, Erich Erlbach (1998). "Beginning Physics II: Waves, Electromagnetism, Optics, and Modern Physics," pg. 50-56
Society
Society is the term to describe human beings together (collective, the sum of their social networks and social interactions). The term comes from the Latin idea of "societas", or the connection between friends or allies (friend or ally being "socius"). It can also mean a specific group of people who interact, as well as a wider society of which they are members. People form societies in order to gain greater benefits as a group than would be possible separately. Many animals beside humans also do this, such as wolves or eusocial insects. Sociology is the name for the study of society.