The bacteria and acid in your stomach break down the things you eat and produce gas as a byproduct. But over 99% of that gas has no odor whatsoever.
The trick is to figure out what's making that 1% stink.
In a 2009 study published in Journal of Chromatography B, Albert Tangerman reviewed all the available literature on flatus (the scientific word for a fart) and performed some experiments of his own to determine what makes farts smelly. Here is a glorious excerpt explaining his methodology:
We developed a bathtub method for quantitative flatus sampling. The volunteers submerged the lower part of their body in a warm bath each time they felt an urge to deflate. The flatus emission was completely sampled by catching all the flatus bubbles in a measuring beaker, which was submerged, fully filled with water and turned upside down before the emission. After some practice, it is very easy to quantitatively sample all the bubbles.
The study implicates methanethiol as the big smell-maker. Gut methanethiol is not super well-understood (outside of the fact that it smells terrible), but it likely comes from processes in your stomach that break down hydrogen sulfide.
Other studies implicate hydrogen sulfide itself as the main driver. Dimethyl sulfide is also a big player in the stink game.
The composition of the bacteria in your gut, as well as your diet, play a big role in determining stank style as well.
Just remember: No two farts are alike.
