"Pinball games were constrained by physical limitations, ultimately by the physical laws that govern the motion of a small metal ball. The video world knows no such bounds. Objects fly, spin, accelerate, change shape and color, disappear and reappear. Their behavior, like the behavior of anything created by a computer program, is limited only by the programmer's imagination. The objects in a video game are representations of objects. And a representation of a ball, unlike a real one, never need obey the laws of gravity unless its programmer wants it to."