The chemical reactions that are fire have what is known as an "activation energy". This means that the process of burning actually has three states, rather than just two. These are: fuel, intermediate, and products. Although energy is released in going from fuel to products, so the fire is hot, and is self-sustaining, the energy level of the intermediate state is actually higher than either of the other two, and so, energy needs to be supplied to initiate the fire, by, for example, a spark.