How does spontaneous combustion happen?

Materials stored in large quantities may spontaneously combust.

This is due to the internal heat caused by oxidation (reaction where electrons are lost, especially when oxygen combines with some other chemical element, or when hydrogen is removed from a compound).

This oxidation does not allow heat to be released into the surrounding air, the material's temperature increases until it reaches its flash point and causes flames.

Around 290 BC, a Chinese text records this phenomenon describing the spontaneous combustion of an oil-soaked stored tissue.