In this episode, Michael Mosley traces our development from birth to adulthood, and reveals that the human brain is so sophisticated it takes more than twenty years to mature. We see how new-born Phoebe makes sense of the world, and how one-year-old Angelina copes with just half a functioning brain. We discover how Moken sea Gypsy children train themselves to see clearly underwater, and meet a Vietnamese girl who speaks 11 different languages. Michael shows his own teenagers remarkable scans which reveal just how many brain connections we lose between the ages of 11 and 20. This remodeling is an essential part of growing up, and helps explain teen behaviour and their tendency to take risks - as illustrated by Stephanie, the world's youngest stock car racer at the age of 13.