Our flatulence is mostly made up of nitrogen, oxygen, carbon dioxide, hydrogen and methane. (This last gas is the flammable agent in a lit fart, btw, but only a third of us produce enough of it to set a toot on fire). Although the aforementioned gases comprise 99 percent of farts, none of them explain the telltale smell. Bacteria in our guts produce the CO2, hydrogen and methane in our farts, and they must also be responsible for other trace gases that stink.
For Provine, the noisiness of farts begs the question: Why don’t we talk out of our butts? No parts of our vocal apparatus evolved specifically for making sound, he points out. “We speak through the same toothy orifice through which we breathe, eat, drink and vomit, and the vocal folds (cords) used for sound production are simply two flaps of muscular tissue that act as a seal to keep food and drink out of the airway when swallowing,” he writes in Curious Behavior. “We ‘play’ this biological instrument to produce speech.” The GI tract has enough analogous morphology to produce sound in the form of farts, but a missing mouthpiece -- or buttpiece, so to speak -- keeps us from effectively shaping sound-producing vibrations into words.