Why do people fart?
As the saying goes: everyone farts. The act of farting, or flatulence, can be a surprising, often embarrassing, potentially stinky, and sometimes outright booming experience that many people would be more than happy to do without. Seriously, though, when it comes down to it, what's farting all about? There must be a good reason, or two, for the, a-hem, unpleasant flatus, a.k.a., gas, right?
Well, for starters, the gas that makes up a fart comes from two sources: most of it originates from the breakdown of food by the digestive system, and the other bit is from swallowed air. In the first case, foodstuffs are broken down by enzymes, stomach acids, and intestinal bacteria in order for the energy in them to be made available to the body. Through the processes of digestion and fermentation, gases can be liberated from what's eaten. In the second case, air that a person swallows from eating, drinking, breathing, chewing gum, and being stressed (among other ways of entry) that's not expelled during a burp travels through the stomach and intestine, transforming into fart-ready mode. In either case, the gases flow down through the rest of the digestive tract and eventually exit the body — Poof! Ping! Pfft! Toot-toot! Kaboom! — expelled as a fart, or farts. And this silent or noisy expulsion of air can help bring about some relief and comfort from a buildup of gas within the lower part of the body.
By the way, the gases that make up a fart are composed mostly of hydrogen, carbon dioxide, and methane. Farts smell obnoxious because of the breakdown of compounds containing sulfur (think rotten eggs). Some foods, such as cabbage, eggs, onions, and meat, contain more sulfur than others, which is why some farts smell worse than other farts.
For more info, a good source worth checking out is Shinta Cho's book, The Gas We Pass: The Story of Farts.Alice!