Author and kindergarten teacher (and MacArthur Genius Grant recipient) Vivian Paley tells the story of an experiment she conducted in her classroom to make children less cruel to each other. She instituted a rule: "You can't say 'You can't play.'" In other words, if two children are playing, and a third child comes over and wants to join them, they can't tell him or her to get lost. They can't reject him or her. This is the cause of unending pain in most classrooms and playgrounds. The experiment was a remarkable and immediate success. (11 minutes)