A fun little electric component that is kind of tricky to get. It is a thin piece of plastic with two leads on it, and when you bend or tap or blow on it, you can get a big, very brief voltage spike. Good for cheap accelerometers. Also somewhat heat sensitive.

I made a raindrop counter with one of these once. It could measure individual drops, and average drop size. I took the idea to the patent office of my university, and they tried to sell it, but no one seemed interested in the idea. Sigh. I later learned that raindrop detectors are common first projects for people that play around with piezo film.