What are some good science demonstrations that you have seen?

Designing a good science demonstration is not easy. It has to be something that takes a relatively short time yet gives an impressive result. The one that most sticks in my mind was a fairly simple one I saw at a science museum. There was an exhibit with a button to press. When I pushed the button, a rock and a feather were released from the top of a vacuum tube. With no air friction, the feather fell as fast as the rock. This really brought home to me the universality of the gravitational constant.