On a German dairy farm, methane produced by the farts of 90 cows built up in a shed and, after a spark, caused a fire in 2014 [source: Kirschbaum]. Yes, farts really are flammable. And not just cow flatulence; human flatus is, too.
Flatulence is a combination of carbon dioxide, hydrogen, nitrogen, oxygen and methane. Two of these gases are flammable -- methane (did the cow story tip you off?) as well as hydrogen sulfide, which means, yes, technically you really can light your farts on fire. (But please don't.)