Fire is the result of a chemical reaction called combustion, a type of oxidation reaction that occurs when a combustible fuel is exposed to a source of heat in the presence of oxygen. The oxidation of the molecules that make up fuel is an exothermic reaction, meaning it releases energy.
When wood is heated to about 300 degrees Fahrenheit, the heat begins to break down the cellulose, releasing volatile gases. When the gases reach about 500 degrees Fahrenheit, the compound molecules begin to break apart and recombine with oxygen to form carbon dioxide and other products. This process is called oxidation, and heat is a side effect of the process. The heat generated by the violent oxidation of wood or other fuels is seen and felt as fire. This heat is often sufficient to ignite other fuels, making fire a self-sustaining process.