Studies of rats and some primates show that rough-and-tumble play among juveniles improves social competence, cognition, and emotional regulation later in life. Most critically, such play makes animals better able to respond to unexpected situations. But not all animals engage in play, and not all animals that play appear to gain these benefits. Using a model developed by Burghardt (2005), the authors argue that there are enabling conditions--such as how behavior systems develop and the presence of surplus resources--that make play-like behavior possible. Once such behavior emerges, other enabling conditions help transform it into more exaggerated patterns of play that can be co-opted for various functions. For species living in complex social systems with an extended juvenility, play has become a tool to refine the control that the prefrontal cortex has over other neural circuits. Such control permits these animals to have more nuanced responses to a variety of situations. In short, the juvenile experience of play refines the brain to be more adaptable later in life.