Spontaneous combustion is a type of combustion that occurs without an external heat source. It happens when a substance with a relatively low ignition temperature, such as hay, straw, or peat, begins to release heat due to oxidation in the presence of moisture and air, or bacterial fermentation, which generates heat. The heat is unable to escape, and the substance self-heats to a temperature high enough for ignition to occur, leading to thermal runaway and autoignition.

Spontaneous combustion can occur in a variety of materials, including oily rags, towels and linen, hay bales, mulch, leaf, manure, or compost piles, and even coal. The possibility of spontaneous combustion is greater if the surrounding air is warm and dry.

There have been anecdotal reports of spontaneous human combustion, where people catch fire with seemingly no external ignition source. However, the scientific consensus is that incidents which might appear as spontaneous combustion did in fact have an external source of ignition, and that spontaneous human combustion without an external ignition source is extremely implausible.

In summary, spontaneous combustion is a real phenomenon that occurs when a combustible material self-heats to a temperature high enough for ignition to occur, without an external heat source. It can occur in a variety of materials, and while there have been reports of spontaneous human combustion, the scientific consensus is that it is extremely unlikely to occur without an external ignition source.