 Why does Purnie start limping?
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. 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. 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.