 Why does Purnie start limping?
It was exactly as he knew it would be: the milky-orange stream had become motionless and its minute whirlpools had stopped whirling; a nearby bee hung suspended over a paka plant, its transparent wings frozen in position for a downward stroke; and the heavy purple fluid overhead held fast in its manufacture of whorls and nimbi. With everything around him in a state of perfect tableau, Purnie hurried toward the ocean. When Purnie had stopped time, the bees—like all the other creatures he met—had been arrested in their native activities, and he knew that as soon as he resumed time, everything would pick up where it had left off. With it under his arm he bounded along a dozen more steps, then stopped abruptly in his tracks. Further down the beach was a pair of two-legged animals poised in mid-step, facing the spot where Purnie now stood. It was time itself that Purnie had stopped, not the world around him. The tripon thought Purnie's feat was superb. It stopped munching long enough to give him a salutory wag of its rump before returning to its repast. Purnie ran from pillar to post, trying to see and do everything at once. Purnie's invitation got nothing more than startled glance from the animals who quickly returned to their chatter. The animals stopped so suddenly that Purnie nearly tangled himself in their heels. As Purnie followed along, a leg shot out at him and missed. Purnie shrieked with joy at this new frolic and promptly stood on his head.
 Purnie is limping because he was shot in the leg.
 Purnie is limping because a petrified log fell on his leg.  Purnie is limping because he was shot in the leg.  Purnie is limping because he tripped on a petrified log.  Purnie is limping because of radiation poisoning.