Big has nearly finished every puzzle in his book of puzzles - there is just one more to do. But it's the hardest puzzle of all: 'What moves backwards and forwards but doesn't go anywhere?' and no matter how hard he tries, Big just can't think of the answer.
Small doesn't care - he thinks puzzles are not exciting. Small says if Big goes outside and plays ball with him right now, Small will help Big with the puzzle later. It's a deal. However, Big still thinks about the puzzle when he is supposed to be playing ball and doesn't join in the game.
Small decides that going on the swing is the perfect activity to take Big's mind off that puzzle. This seems to work because Big stops thinking about the puzzle for a moment and enjoys swinging, while Small gets ready to give him a huge push. Suddenly, Big hops off the swing: he's just solved the puzzle!