Once I observed a group of children engaged in superhero play. 'Superman' was encouraging the 'baddies' to attack him, but the sight of his playmates charging across the grass prompted a change of heart and he suddenly called out, 'Hold on. One at a time!' The incident raises important questions about aggression in children's play. Were the 'baddies' being aggressive? Was 'Superman' afraid? Or was the incident part of a well-ordered game, in which a speedy rule change...