  If a tennis ball is held above a basket ball with their centers vertically
aligned, and the balls are released to collide with the floor, the tennis ball
may rebound at a surprisingly high speed. We show in this article that the
simple textbook explanation of this effect is an oversimplification, even for
the limit of perfectly elastic particles. Instead, there may occur a rather
complex scenario including multiple collisions which may lead to a very
different final velocity as compared with the velocity resulting from the
oversimplified model.
