Spontaneous combustion is when something suddenly bursts into flames without anyone lighting it on fire. Instead of needing a match or spark, the material heats itself up through chemical reactions until it reaches its ignition temperature and catches fire all on its own.

This happens most often with oily rags left in piles. When oil slowly reacts with oxygen in the air, it produces heat. If that heat can't escape because the rags are bunched together, the temperature keeps rising. Eventually, the pile gets hot enough to ignite. Farmers worry about spontaneous combustion in hay bales that were stored while still damp: the hay ferments, builds up heat, and can set an entire barn ablaze.

Coal piles, compost heaps, and sawdust can also spontaneously combust under the right conditions. The key ingredients are always the same: a material that generates heat through chemical reactions, insulation that traps that heat, and enough time for temperatures to climb high enough for a fire.

Despite spooky stories, people cannot spontaneously combust. Those tales are myths. But materials can, which is why professional workshops and farms take precautions with things like oily cleaning rags, storing them in special metal containers where air can circulate and heat can escape safely.