Raise balls up to an altitude of around 1 m and let the balls drop freely.
How does it work
As the balls rebound an exchange of energy and momentum occurs between them. Because, in this particular example, the rebound is resilient, the mechanical energy as well as the momentum of both balls is conserved. That is why, in the course of the experiment, the upper ball, which has a smaller mass, will gain extra energy at the expense of the lower ball with its greater mass and, as a result, will bounce back to a higher altitude.