“Build a man a fire, and he'll be warm for a day. Set a man on fire, and he'll be warm for the rest of his life,” author Terry Pratchett once wrote.

Spontaneous human combustion is one of the more vexing medical mysteries in existence. A person is found burned to ashes, with no apparent ignition source nearby. Usually objects nearby are unharmed, and sometimes parts of the body -- usually the legs -- are left relatively intact.

One early account of this phenomenon hails from 17th-century Danish physician Thomas Bartholin, who related the case of Milan resident Polonus Vorstius, who mysteriously combusted after an evening of wine-drinking in 1470. Other victims like poor Vorstius had also been drinking alcohol before they burst into flames, which led to a heavy dose of moralizing from Victorian scientists of the time, warning of the dangers of drunkenness.

Modern scientists have speculated that a tiny flame, like a burning cigarette, could fall on a sleeping victim and burn them enough to melt subcutaneous fat that would be absorbed by the person’s clothing and act as a wick to keep the flame going. Most of the cases of spontaneous combustion have involved people with poor health, the elderly or the obese that would probably have difficulty moving if they caught fire.

But still, what can set off the fire? Nineteenth-century German chemist Justus von Leibig, also known as the father of the beef bouillon cube, ruled out alcohol as the cause of spontaneous combustion in his experiments. Even after injecting rats with alcohol, he could not set them on fire, according to independent researcher Brian J. Ford.

Ford put his own theory about spontaneous combustion to the test by marinating pork tissue in acetone, a highly flammable chemical that can sometimes accumulate in the human body thanks to ketosis, when energy-starved cells start using fatty acids instead of glucose for energy. One of the products of this chemical process is the molecule acetyl-CoA, which can be converted to acetoacetate in the liver, and then be further transformed into acetone.

When Ford and his team set mannequins made of acetone-soaked pork tissue on fire, the remains they found were similar to those found at the scene of alleged spontaneous combustion victims: a pile of ash with limbs sticking out. They reported their results in the journal The Microscope.

“The legs remain, we think, because there is too little fat for much acetone to accumulate,” Ford wrote for the magazine New Scientist this past August.

Ketosis can be caused by alcoholism -- meaning the moralistic Victorians were actually not all wrong -- but by other things too, including low-carb dieting. But given that there are only slightly more than 100 cases recorded throughout human history, statistically it’s not something most alcoholics or carb-cutters should worry about.

“On the other hand, there would be an argument against people with ketosis wearing synthetic fibers on dry days and a new argument to give up smoking,” Ford wrote.