When you stick out your tongue to a newborn baby, the newborn baby will do the same. This is thought by some scientists to be one of the earliest manifestations of developing “mirror neurons,” the physical basis of empathy and mutual understanding. Mirror neurons cause you to experience things that are happening to others as if they were happening to you. When a baby hears another baby crying, its mirror neurons will react, making it experience the same emotion as the other baby. That's why crying is contagious from one baby to another.