Since the days of ancient Greece, thinkers have grappled with the mystery of why humans and other animals play. Survival is hard, so why waste energy? Over the years, theories have ranged from Plato’s simple picture of play as skill rehearsal to ever more esoteric ideas about surplus energy, Freudian catharsis, and vestigial “primitive instincts.” These days, developmental psychologists mostly see play as a way of gathering information about the world and our place in it. But there’s something missing from this picture, according to Marc Malmdorf Andersen, a cognitive scientist at Aarhus University in Denmark. “Yes, it’s about learning about the world,” he says. “Everyone agrees that play is really good for that. But no one really deals with the fun aspect.…