Here's one you probably never thought to ask about before. You're in the woods building a campfire. You pile up some dry kindling and light a match. Hold the match under the kindling and, sure enough, the kindling starts burning.
Okay, so why is that? You held a burning stick up to a non-burning stick. Why do you now have two burning sticks?
The answer is in the nature of fire. If you could look at a flame with super-penetrating vision, you'd see that on an atomic level what you have is the rapid combination of carbon and hydrogen atoms in the wood and oxygen atoms in the air. Given a chance, these atoms will link up to form molecules, such as carbon dioxide and hydrogen dioxide. When this happens, energy is released. That energy causes the light and heat that we see and feel when sitting around a campfire.
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.