Okay, so if the wood is full of carbon and hydrogen, and the air is full of oxygen, why doesn’t the kindling burn all by itself? Remember we said that given a chance, these atoms will link up to form molecules. But carbon, for example, won’t combine with oxygen unless it’s given enough energy to jumpstart the process. Physicists call this the “activation energy.” You add that extra activation energy to the wood by holding a match to it. Once the atoms in the wood have been prompted to start combining with oxygen, the energy that they release will act as activation energy for more combination. Now the process runs by itself. The kindling has caught fire.