MY SON MARK HAD PRACTICED gymnastics for several years and he was ready to learn a double back flip on the trampoline. His coach buckled a spotting belt around Mark's waist and clipped on two ropes that stretched upward to two pulleys on ceiling beams some thirty feet apart and then rejoined and dropped back down to the spotter. If Mark was about to fall on his head, the coach would pull on the rope and suspend him in the air. Mark practiced doubles safely for several weeks in the spotting belt and proudly reported to me how well he was doing.

Then one Saturday he came home with bruises on his head and hip and a disquieting story. In the middle of a double flip, when Mark's coach pulled on the ropes, one of them came unclipped from the steel ring on the belt. Since the remaining rope ran to a ceiling pulley many feet to the side of the trampoline, Mark swung pendulum-like sideways off the trampoline, where he met a wall.

Until this episode, Mark had placed complete confidence in the spotting ...