A child's very first word is a miraculous sound, the opening note in a lifelong symphony. Most parents never forget the moment. But that first word is soon followed by a second and a third, and by the age of three, children are typically learning ten new words every day and speaking in complete sentences. The process seems effortless, and for children, it is. But how exactly does it happen? How do children learn language? And why is it so much harder...