The catalyst that welds these two strangers together is interactivity. All play requires interaction of some sort. But as we grow, bouncing balls lose their fascination, and we seek out interactive partners capable of richer behavior. Some of us, mostly adolescent males, get sidetracked and fixate on computers as interactive partners. But most of us seek out other people as interactive partners, for the equality of interaction of human intercourse provides the most challenging and complete form of play.
Interaction can only take place where there is a perceived disrepancy of volition.
The various angles, energies, and spins with which the ball bounces back are perceived by the child as manifestations of its volition. After much experimentation, the child induces the laws of physics that determine the ball's behavior. This triggers a fundamental shift in the child's perception of the ball. It is no longer an agent with free will, capable of interacting with the child, but instead an inanimate object reacting according to understandable rules. The ball is left lying on the floor as the child seeks out new agents with which to interact.