The piston

The piston is an amazingly simple, ridiculously useful and successful mechanism that, together with a connecting rod and crankshaft, converts linear motion to rotational motion.

This simple system is basically what allows us to take a force produced by, say, steam or an explosion and make a wheel turn. Stick a few of these together and time the rotation of each correctly, carefully controlling the explosions and the exhaust, and you have an internal combustion engine that powers and moves our cars.

There are a lot of amazing things to me about it, but I am still impressed with how effectively it takes something moving back and forth and makes something spin around and vice versa.