Hypothesis

For the video with the basketball being dropped with the tennis ball on top, I think the reason for letting a small ball drop on top of a larger ball make such a dramatic different in bounce because with the momentum and force of the basketball after its bounce has gone back up, it pushes against the tennis ball, pushing it up with much more force. And because the basketball has such a lager mass compared to the tennis ball, it's force being pushed onto the tennis is much greater then the tennis ball's force when it bounces back up with it's own force. So it bounces higher then it would if it were to just be dropped because it's being pushed up by a larger force then it's own. (The basketball)