The adolescent brain does not grow so much as it edits. Through early childhood the brain overproduces synapses—the connections between neurons—laying down far more circuitry than it will ever use. Then, in the second decade of life, a great winnowing begins. Connections that fire often are strengthened and preserved; those that fall silent are dismantled and cleared away. Neuroscientists call this synaptic pruning, borrowing the gardener's word, and the metaphor is apt in a way that is easy to overlook. A gardener does not prune to harm the plant but to shape it, cutting back the superfluous so that the remaining growth can flourish. So too the brain: by eliminating the connections it does not need, it grows faster and more efficient in the ones it keeps. The process can feel, from the outside, like loss—the toddler's dazzling capacity to learn any language on earth narrows, by adolescence, to fluency in a few. But this narrowing is not decay. It is specialization, the price a brain pays to become expert at the particular world it happens to inhabit.