2 or 3 balls of different sizes. A basketball, tennis ball, and a golf ball, for example.
This takes a little practice but it is worth it. Try it with two balls first.
1. Just bounce both of the two balls by themselves to see how high they bounce.
2 Now stack the two balls one on top of the other. The bigger ball on the bottom.
3. Drop both balls at the same time. Hopefully they will fall together.
4. Watch the balls bounce! The ball on top will bounce much higher then it did by itself.
5. Now try it with three balls.
This is a little easier if you only drop the balls a couple of feet. It works because the momentum of the heavy, larger ball gets transferred to the smaller ball and sends it skyward!