Few phenomena in childhood are as compelling—and mystifying—as play. We review five proposals about the relationship between play and development. We believe each captures important aspects of play across species; however, we believe none of them accounts for the extraordinary richness of human play or its connection to distinctively human learning. In thinking about play, we are particularly struck by the profligacy with which children set seemingly arbitrary rewards and incur unnecessary costs. We suggest that researchers take the seeming inutility of play seriously and consider why it might be useful to engage in apparently useless behavior. We propose that humans’ ability to choose arbitrary costs and rewards allows us to pursue novel goals, discover unexpected information, and invent problems we would not otherwise encounter. Because problems impose constraints on search, these invented problems may help solve a big problem: how to generate new ideas and plans in an otherwise infinite search space.
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