Combustion of a liquid fuel in an oxidizing atmosphere actually happens in the gas phase. It is the vapour that burns, not the liquid. Therefore, a liquid will normally catch fire only above a certain temperature: its flash point. The flash point of a liquid fuel is the lowest temperature at which it can form an ignitable mix with air. It is the minimum temperature at which there is enough evaporated fuel in the air to start combustion.

The same principles apply to the combustion of solids. 

The combustion progression of a solid fuel starts with the drying and removing of any moisture in the fuel. Once dried, the fuel will start to heat up and release volatile organic gases. Then, the volatilized organic gases will ignite and combust, releasing energy. This process will continue until the energy required to release volatile organic gases is not adequate or the hydrogen/carbon atoms are completely consumed.