For more than 50 years researchers have used a very simple method to investigate learning in infancy. They place a baby into a crib with a mobile suspended above it. Then a scientist ties one end of a string to the mobile and the other to the infant’s foot. If the baby moves, the toy will, too. Over multiple sessions, scientists can observe as the infants learn and recall a simple cause-and-effect interaction: kick a foot, and the mobile moves.