scientists have learned that free play isn’t just something children like to do—it’s something they need to do. Play keeps kids physically active, all the more important at a time when some 20% of American children are obese—more than triple the percentage from the more play-friendly 1970s. (Early activity habits matter—a 2005 study in the American Journal of Preventive Medicine found that the most active 9-to-18-year-olds remained the most active later in life.) It also exercises their minds and their creativity. More than anything else, play teaches children how to work together and, at the same time, how to be alone. It teaches them how to be human.
Yet one of the best ways to understand why children play is to look at the behavior of young animals. Primates and many other animals play as juveniles, usually with a characteristic gait or signal that demonstrates to other animals that their activities—which can seem aggressive—aren’t meant to be taken seriously, just as children might smile as they play-fight. Play among animals is more conditional on the environment than it appears to be among children—during periods of drought and food scarcity, young animals will cease playing. But play does have a major impact on the brains of animals—and researchers believe it may have a similar impact on the brains of human children.