Fortunately--ah-ha!--what's that glinting in the sunset over there? It's a piece of steel! And this stone? Why, it's flint! You probably know what to do from this point. Strike the flint and steel together to make sparks, then use those sparks to start your flame. Humans have been starting fires this way for centuries. Why does it work?

The key to this whole thing is the steel. If you strike steel against anything hard enough and fast enough, it will make sparks. For example, horseshoes make sparks as they gallop over a gravel path. And have you ever seen a car driving with its tailpipe hanging down, showering the road with sparks? It's the steel in the pipe that makes those sparks, even though the road is not flint. Flint is the traditional choice for this fire starting method only because real flint, silicon dioxide, is especially hard.

When you strike the flint and steel together, this does two things at once. First of all, it knocks small particles of steel away from your steel bar, kind of like tiny steel filings. Second, it creates a lot of heat from the friction as the two materials rub together. This heat is enough to cause the steel filings to literally start burning. As they fly through the air, they combine rapidly with atmospheric oxygen, flaring up as the spark you can see. You start your fire with these tiny, flying pieces of flaming steel.  

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A Moment of Science^®