I remember watching a TV show about this a few years ago and the best explanation was the wick effect. That is, a source of flame (such as a burning cigarette), burns the clothing of the victim in one area, splitting the skin and releasing fat from under the skin. The fat is then absorbed into the burned clothing, and acts like a candle wick. The burning can continue for as long as the fuel is available. This hypothesis was successfully demonstrated on the show with a pig. With that being said, some debate whether this process can occur in humans and cases continue to pop up now and then.
If you have not heard of a specific case, below is a report of a case of spontaneous human combustion reported several days ago by Nick Collins at The Telgraph.
Man 'spontaneously combusts'
Spontaneous human combustion has long been the stuff of fiction, endorsed by eccentric scientists and employed by novelists including Charles Dickens as a convenient plot twist.
But yesterday the most unlikely cause of death, in which people burst into flames without any external source of ignition, was given official sanction when Irish coroner found a pensioner had burned to death for no apparent reason.
Michael Flaherty, 76, was found dead at his home in Galway last December after a neighbour heard the smoke alarm in his house go off in the middle of the night.
But while his body had been burned to cinders, fire officers who attended the scene were astonished to find nothing else had been damaged apart from the floor below him and a patch of ceiling above.
There were no signs of any devices which could have ignited the body, and no indication of foul play, officials said – Mr Flaherty's body appeared to have simply cremated itself.
Officers who attended the scene claimed they had never seen anything like the extraordinary case, and the inquest heard fire officers were unable to give any explanation for what sparked the blaze.
Recording his verdict, west Galway coroner Dr Kieran McLoughlin was left with little option but to become the first coroner in the country's history to record the unusual verdict.
He said: "This fire was thoroughly investigated and I'm left with the conclusion that this fits into the category of spontaneous human combustion, for which there is no adequate explanation."
Spontaneous human combustion was a phenomenon first described by Victorian doctors, who suggested the body could suddenly go up in flames as a divine punishment for alcoholism.
Other explanations for the unexplained combustion of the body include the influence of ghosts or other paranormal entities, the production of unusual concentrations of gas, or external factors like cigarette sparks.
In the 1850s Charles Dickens, the novelist, attracted controversy after Krook, a rag and bottle merchant, spontaneously combusted in Bleak House.
The mystique of the theory is heightened by the striking similarities between documented cases. In many instances the body is found reduced to ash while the arms, hands and legs remain, and in several others the victim is completely consumed while nearby objects such as furniture remain untouched.
In one example, a Welsh policeman who found the victim's body noted that the fire appeared to have come from within her abdomen.
The latest case bore many of the hallmarks of the classic case – the victim was found on his back by the fireplace, with his head intact but the rest of his body entirely consumed.
Fire experts said the evidence suggested the fire had not been the source of ignition.
Bob Rickard, of the paranormal magazine the Fortean Times, told the Telegraph: "It has become rare now, I have not heard of a case for a couple of decades. But what is even more interesting to me in this case is it is the first time I can remember that a coroner has come out and announced a verdict of spontaneous human combustion.
"Normally they try to leave an open verdict or try to express it in some other way."
Mr Flaherty's family said they were satisfied with the investigation, the Irish Independent reported.