Flatulence, also called fart or flatus, is when intestinal gas leaves the anus. This often happens with a loud sound, caused by the speed and amount of gas released, the vibration of the anal sphincter, and occasionally by the closed buttocks.
In mammals like humans, the gases come from two sources: swallowed air (for example, while eating, talking or drinking) and gases naturally created by bacteria in the body during digestion. Nitrogen is the main gas released (20-90% in total), followed by hydrogen, carbon dioxide, oxygen and methane. All these gas are odorless; the bad odor often associated with flatulence is caused by compounds that contain sulfur, such as methanethiol, hydrogen sulfide and dimethyl sulfide. Because methane is highly flammable, some people can light up their flatulences, but not all humans produce flatus containing methane. The gas that doesn't come from swallowed air is produced by the bacteria that live in the intestine; these bacteria help us digest the food we eat by feeding on it, and this causes gas to be produced. Foods that produce large amounts of gas are usually high in carbohydrates or polysaccharides, such as beans, lentils, cheese, pasta, vegetables and fruit. Some people can't digest milk, and because of this they can get flatulence when they drink it. Some other people swallow a lot of air when they chew gum, smoke, drink carbonated drinks, eat too fast, wear loose dentures or just because they're nervous; this causes them to release more gas and is known as aerophagia.
Intestinal gas is brought to the rectum in the same way as feces, so it causes a similar feeling of urgency and discomfort. Researchers have discovered that the sensitive nerves around the anus exist to help us tell the difference between flatus and feces; sometimes, though, a person can get confused and accidentally defecate while trying to release gas. This is colloquially known as sharting and can leave stains in a person's underwear. There are many slang terms for the passing of flatulence, such as farting, pooting, tooting, passing gas, breaking wind, and cutting the cheese.
All people produce and release flatulence, on average 14 times every day. This can happen accidentally, for example while coughing, sneezing or even during an orgasm, but people can also pass gass at will; this is often done by bearing down on the stomach and relaxing the anal opening, or by raising a leg and pushing. Flatulence is completely normal and a sign of good health, but because of its odor, many people feel too embarrassed to pass gas in front of strangers and will try to hold it until they are alone. This has caused flatulence to become a taboo in many occasions; at the same time, most people think flatulence is funny, so it is often a source of humor in jokes, artworks, books and movies. Not only humans pass gas: in fact, all animals expel flatus, including many invertebrates. Even birds, worms, ants, fishes and reptiles produce flatus. Because of their different diet, the flatus of carnivores, such as dogs and cats, is usually smellier than that of herbivores, such as cows.