Rebound

lift the ball. make its second bounce reach the flag.

drop a tennis ball so its second bounce reaches a flag, and watch where each bounce's energy goes

A tennis ball keeps about 55% of its height each bounce: the rules say one dropped from 254 cm must come back up 135 to 147 cm. The rest of the energy goes into the thermal store of the ball and floor, a little of it by sound, so the second bounce is about 30% of the drop. The total never changes.

Concepts: energy, restitution, conservation.

An interactive you play with one thumb, on Wundr.