Fire is a chemical reaction in which a combustible fuel reacts with oxygen to release large amounts of thermal energy. Many atoms bind very strongly with oxygen atoms and these fuel atoms release energy when they bind with oxygen. Initiating these combustion reactions normally requires some thermal energy to get started. This starting energy is known as activation energy. That's why you have light the fire—you must provide the activation energy. After that, each oxidization reaction produces the activation energy needed to start another oxidization reaction and the fire keeps itself going until it has consumed all of its fuel.