Two balls are stacked on top of one another in a double-ball bounce experiment. The lower ball strikes the ground, bounces back, and slams into the upper ball.

This illustrates the conservation of momentum principle in physics. It entails holding two balls so that the tennis ball is directly on top of the basketball, one large (a basketball or football) and the other little (a tennis ball). They fall simultaneously from a height of around a meter. The basketball bounces when it lands, and the smaller ball strikes the basketball while it is moving upward, causing it to jump far higher than it would have otherwise.

The double-ball bounce experiment is significant because it is used to show that the rigorous rules of physics can produce counter-intuitive effects.

In a previously done double-ball bounce experiment, the experimenter observed that the little ball bounced back with a velocity of two times the velocity it had when it was dropped. Assuming that it was a fully elastic collision and that the large ball is significantly more massive than the small one. The observer measured the little ball's velocity as three times the velocity it had when it was dropped.