When two molecules meet under the right circumstances, they may exchange electrons in ways that change both molecules into new kinds of molecules. While they're doing that - reacting to each other - they may also release some electrical energy in the form of heat or light. This is what happens whenever there is a fire. The earliest fires were stars, where when hydrogen atoms meet under a lot of pressure from gravity, they merge together into helium atoms and let off some extra energy - that's the sunshine we get from our Sun.
The same kind of thing happens in a forest fire, or when you light a candle or a match. Candles are made of hydrocarbon molecules (sometimes oil, sometimes beeswax, sometimes tallow from animal fat), and matches are made from hydrocarbon molecules (wood). When they get hot enough, these hydrocarbon molecules react with the oxygen in the air. The heat can come from friction, like when you strike a match, or from another fire, like when you hold the match to the candle, or from lightning that starts a forest fire, or from focused sunlight. When the hydrocarbon molecules reach 300 degrees Fahrenheit (150 degrees Celsius), the reaction begins.
During the reaction, the molecules come apart and recombine into carbon, carbon dioxide, smoke, and water. But the reaction leaves a little energy left over, and that's the heat and light of the fire.
Learn by Doing - starting a fire
A slower kind of combustion reaction: rust
Another slow kind of combustion reaction: digestion
To find out more about chemistry, check out these books from Amazon.com or from your library:
Atoms and Molecules (Usborne Understanding Science) , by Roxbee-Cox (1991). Ages 9-12. A basic introduction.
Chemistry (DK Eyewitness Books), by Ann Newmark and Laura Buller (2005). DK is a respected series.
Simple Chemistry, Grades 4-6, by Evan-Moor (2008). Ages 9-12. Good simple projects to illustrate concepts.