You're lost in the wilderness and night is approaching. You want to start a fire, but oh no, you're out of matches. Wait, what's that glinting in the sunset over there? It's a piece of steel, and mere feet away from a piece flint, what luck!
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. Same goes for a car driving with its tailpipe hanging down, showering the road with sparks. It's the steel in the pipe that makes those sparks.
When you strike the flint and steel together, it 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.