Spontaneous Human Combustion

The physical possibilities of spontaneous human combustion are remote. Not only is the body mostly water, but aside from fat tissue and methane gas, there isn't much that burns readily in a human body. To cremate a human body requires a temperature of 1600 degrees Fahrenheit for about two hours. To get a chemical reaction in a human body that would lead to ignition would require some doing. If the deceased had recently eaten an enormous amount of hay that was infested with bacteria, enough heat might be generated to ignite the hay, but not much besides the gut and intestines would probably burn. Or, if the deceased had been eating the newspaper and drunk some oil, and was left to rot for a couple of weeks in a well-heated room, his gut might ignite. And in each of these ludicrous scenarios additional oxygen would have to be introduced. These possibilities are so farfetched that I have no reason to believe they, or anything like them, has ever occurred.

Taken from The Skeptic's Dictionary