Alison Gopnik understands babies and children as the R&D division of humanity. From her cognitive science lab at the University of California, -Berkeley, she investigates the “evolutionary paradox” of the long human childhood. When she first trained in philosophy and developmental psychology, the minds of children were treated as blank slates. But her research is helping us see how even the most mundane facts of a toddler or a teenager — from fantasy play to rebelliousness — tell us what it means to be human.