Why do farts smell?

Gut gas is made of swallowed air plus gases produced when bacteria in the large intestine ferment food we cannot fully digest. Around 99 percent of it is odourless nitrogen, carbon dioxide, hydrogen, oxygen and, in some people, methane. The stink comes from less than 1 percent of the mix: sulfur-containing gases such as hydrogen sulfide, which smells like rotten eggs, and methanethiol. Foods rich in sulfur, including eggs, meat, cabbage, broccoli and onions, give bacteria more raw material. A healthy person typically passes gas around 14 to 23 times a day.