Babies are born listening and learning. Their language environment makes all the difference to their development. Language-rich environments literally build babies' brains. Serve and return interactions that adults and older kids have with baby in their first three years are building neural connections that will be with them for life. A baby's brain doubles in size in their first year of life. This demonstrates the tremendous growth they are experiencing every day. By their third birthday, their brain is 80% of the size of an adult brain.