http://en.wikipedia.org/wiki/Flatulence
and see also:
http://en.wikipedia.org/wiki/Professional_farter
http://en.wikipedia.org/wiki/Toilet_humour
http://en.wikipedia.org/wiki/Lighting_farts

all gripping reading.

Flatulence ignition or "fart lighting" is the practice of setting fire to the gasses produced by flatulence. It is practised primarily among young men, but discouraged for its potential for causing injury. Lighting such gas can result in burns or explosions. Clothing or hair may catch fire and sensitive tissues can be damaged.

Such experiments may occur among young men on camping trips or in single-sex group residences such as dormitories or fraternity houses, but the flammability of bodily gases has caused serious problems in the operating room and also in slaughterhouses.

At least one patient is known to have exploded while undergoing cauterization of a rectal polyp. An electric spark ignited the patient's intestinal gases, resulting in a six-inch (15-cm) hole in the patient's large intestine. However, this was sewn up, and the patient recovered [1].