Stories of spontaneous human combustion (SHC) date as far back as the 1400s; the knight Polonus Vorstius was said to have drunk two ladles of wine before vomiting a flame and becoming consumed by fire. But one of the earliest written accounts came in 1633 when Danish anatomist Thomas Bartholin described how a woman in Paris "went up in ashes and smoke" while she slept but left her straw mattress unaffected by the flames. SHC really got its time in the spotlight when Charles Dickens used it to kill off a character named Krook in his novel "Bleak House." News accounts of people becoming engulfed in flames, seemingly without an outside cause, have also cropped up many times in the last century.

Most of these examples have a few things in common: the person is usually immobile — often because they're passed out drunk or on other drugs, but sometimes it's due to a physical disability. The body is usually completely reduced to ashes, but sometimes the extremities remain intact. And as we mentioned before, the person's surroundings are often unmarred by smoke or fire, save for a few mysterious grease stains on the walls or ceiling.

How would that even be possible? Proponents have a few theories. Some say that it happens when digestive enzymes ignite a buildup of methane in the intestines. That doesn't explain why most victims experience the most damage on the outside of their body, not the inside. Other, more far-out hypotheses say it's a result of too much static electricity in the body, some external geomagnetic force, or even a new subatomic particle called the pyroton. But so far, there hasn't been any good science to back up these ideas.

But even if a flame came from some other source — an electric spark or a lit cigarette, for instance — that doesn't explain why victims of so-called SHC burn so completely. Science has a possible explanation, though. It's called the wick effect. Fair warning, this gets pretty gruesome.

A candle is made up of a wick surrounded by wax — an ignition and a fuel source. When you light the wick with an external flame, the heat melts the wax closest to the wick. That liquid wax is then drawn up the wick by capillary action until it reaches the flame and is vaporized, creating more heat that keeps the cycle going. That's why a candle can burn for so long.

The human body can be thought of as an inside-out candle: flammable clothing or hair acts as the wick, and body fat acts as the fuel source. As heat from the flame reaches the fat, it melts and soaks into the "wick," vaporizing and creating more heat to keep the body burning continuously. All the fuel the fire needs is right there, which could be why a victim's surroundings are left untouched save for a few grease stains — the leftovers from burning fat. We told you it was gruesome.

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Former private investigator Joe Nickell has spent his life investigating mysteries just like this one with a skeptical, evidence-based eye. Dive into his findings about spontaneous human combustion and other eerie phenomena in "Real-Life X-Files: Investigating the Paranormal." We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.