Paul W. Frankland of the Hospital for Sick Children in Toronto and his colleagues suspected that the dramatic growth of neurons in the infant brain might interfere with the formation of memories. To test this idea, they ramped up neuron production in adult mice right after they learned something new. Unlike youngsters, adults grow neurons only slowly. But add exercise or drugs such as antidepressants, and neurons proliferate. When adult mice exercised after learning that a certain cage delivered a foot shock, the memory of the painful lesson diminished. Then the team turned to infant mice, whose brains naturally grow lots of neurons. These young mice rarely remember the foot shock for more than a day. When the researchers used a drug to slow the infants’ neuron growth, the mice retained the memory for a week.