http://en.wikipedia.org/wiki/Exploding_cow
The exploding cow is the alleged practice of fart lighting with cattle. As a cow produces about 280 liters of methane-rich flatulence each day some people believe that by using a tube inserted into a cow's anus, one can ignite the gas and cause the cow to explode.
Methane (CH4) and hydrogen (H2) are the main flammable ingredients of bovine intestinal gas. The oxidation of one part of methane requires two parts of oxygen. In contrast, burning hydrogen requires much less oxygen:
CH4 + 2O2 ¨ CO2 + 2H2O
2H2 + O2 ¨ 2H2O