What is Fire?
The ancient Greeks considered fire one of
the major elements in the universe, alongside water, earth and air. This grouping makes intuitive sense: You can feel fire,
just like you can feel earth, water and air. You can also see it and smell it, and you can move it from place to place.
But fire is really something completely different. Earth, water and air are all forms of matter -- they are
made up of millions and millions of atoms collected together. Fire isn't matter at all. It's a visible, tangible side effect of matter changing form -- it's
one part of a chemical reaction.
Typically, fire comes from a chemical reaction between oxygen in the atmosphere and some sort of fuel
(wood or gasoline, for example). Of course, wood and gasoline don't spontaneously catch on fire just because they're surrounded by oxygen.
For the combustion reaction to happen, you have to heat the fuel to its ignition temperature.