Propellers, often shortened to "props," are sometimes called screws—and it's easy to see why. To push a screw into the wall, you apply a clockwise turning force to the head with your screwdriver. The spiral groove (sometimes called a helical thread) on the screw's surface converts the turning force into a pushing force that drives the screw into the wall and holds it there. But suppose, for a moment, that you wanted to keep on going...

If you were a beetle and you wanted to move through an infinitely long wooden wall, you could use a screw thread on the outside of your body to do it. You wouldn't need a screw running along the whole length of your body: you could manage with just a little thread on your head—a kind of screw cap—to bite into the wood in front of you. Now suppose you were a fly, not a beetle, and you wanted to go through air rather than wood. There's no reason why you couldn't use a screw thread in exactly the same way to pull you through the sky. In effect, you'd be a fly with a propeller—and that's pretty much what the first airplanes were. Planes took to the sky when the Wright brothers figured out how to combine engine-powered propellers and wings so they could go forward and upward at the same time.

A propeller is a machine that moves you forward through a fluid (a liquid or gas) when you turn it. Though it works the same way as a screw, it looks a bit different: usually it has two, three, or four twisted blades (sometimes more) poking out at angles from a central hub spun around by an engine or motor. The twists and the angles are really important.