Three conditions must exist before a fire can be made. There must be a fuel or a substance that will burn. The fuel must be heated to its ignition temperature. This is the lowest temperature at which combustion can begin and continue. Finally, there must be plenty of oxygen, which usually comes from the surrounding air.
Fuels are of three classes, solids, liquids, and gases.
Coal and wood are examples of solids. Oil and gasoline are liquid fuels. Natural gas and hydrogen are gaseous fuels. The burning of a solid fuel often depends on the form of the fuel. For example, you may not be able to light a large log with a match, but a small twig from the same tree may catch fire easily with the same match.