Spontaneous human combustion is when a person's body catches fire all by itself, without any outside sources of heat or flames. It's extremely rare and there's still a lot of debate about what exactly causes it.
Some people think that it has to do with the chemicals in our body - like if someone had a lot of alcohol or other flammable substances in their system, that could be a factor. Others think that it might have to do with energy from something called "the human aura" - which is kind of like a glowing field of energy that some people believe we all have around us.
But most experts don't really know for sure what causes spontaneous human combustion. It's still a bit of a mystery!