(Thanks to Wikipedia)
Fire isn't the only kind of combustion reaction. Some kinds of metal, if you leave them in contact with oxygen, will also react with the oxygen, but much more slowly, and without making a flame. We call the result rust.
Like burning grass, the combustion reaction between iron and oxygen that produces rust, or iron oxide (a molecule containing both iron and oxygen atoms), also produces heat. In fact, it produces the same amount of heat as the fire, but spread out over a longer time, so that the iron oxide or rust also cools off at the same time that it is heating up.
When iron is exposed to oxygen in the air, the two combine very slowly to make iron oxide, or rust. But if the iron and oxygen molecules are also touching water molecules, the water molecules speed up this reaction and make the iron rust a lot faster.
Black-figure vase by Exekias (Athens, ca. 540 BC)
One special kind of rust reaction happens inside a pottery kiln when you heat up a clay pot and there is some iron in the clay. The iron in the clay reacts with the oxygen inside the kiln and turns red. This is how Greek vase-painting worked.
Learn by Doing - Rust
Another project with rust
Another slow kind of combustion reaction: digestion
A faster kind of combustion reaction: fire
To find out more about chemistry, check out these books from Amazon 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.