the neuronetwork is threading out and even now the nerve signals are traveling more than 150 miles an hour. at birth, nearly all 100 billion nurones of the human brain are already in place. but the brain only weighs about 25% of what it will later on. it's about to embark on its fastest growing period, equal grouping in size by the time a child is just in preschool. by age 6, the brain is 90% of its adult size. during that burst of growth, 700 new connections are formed every second. as we gain the capacity to smile around 2 months. to talk, usually around a year. and to dress ourselves, around the age of 3. in those early years, in fact throughout our lives, the brain changes through experience, learning to speak, taking those first steps, understanding colors and shapes, forming novel thoughts. but a certain nurones are used more frequently, other unused nur ones go away. it's a process called pruning. and almost anything can shape us in those baby and toddler years. first words, first ice cream, first tv show, first argument for better or worse. and here with me now, dr. rosemary t