In one locked room, the kids saw the little boy they had seen bouncing the ball outside. What was different about him?

The little boy was different because each time he bounced the ball he would scream as if he was in pain.

In the room a little boy was bouncing a ball. He was bouncing it in rhythm, and the walls of his little cell seemed to pulse with the rhythm of the ball. And each time the ball bounced he screamed as though he were in pain.