Wood, leaves and plastics do not themselves burn. Heated above roughly 200 to 300 degrees Celsius, they decompose chemically without needing oxygen, a process called pyrolysis, which drives off flammable vapours such as carbon monoxide, methane and heavier tars and leaves behind a carbon-rich char. The flame you see sits in the gas phase, a millimetre or more above the surface, where those vapours mix with air and burn; the solid below is being cooked, not combusted. The rule this gives you is that a solid fuel must first be heated enough to gasify, so anything that steals heat from the surface, above all evaporating water inside the fuel, delays or prevents flaming altogether. The confusion it resolves is the intuitive picture of a flame eating into a solid: the flame is fed by a gas stream, and it survives only while the surface stays hot enough to keep producing that stream. It also explains the smell and visible haze that precede flames, which is pyrolysis product escaping before ignition. After this Concept you can explain why a fuel must be heated before it will flame, and what stops it.
Pyrolysis is the thermochemical decomposition of solid fuel into flammable gases through the application of heat alone, without requiring oxygen or the presence of flame; it is distinct from combusti…