Fire is
a visible, tangible side effect of matter changing form. It is 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. This is done by matches, focused light, friction, lightning,
something else already burning.
