If we walk down a street where we were once bitten by a dog, we feel fear. This is because our brains are great at creating associations. The street and the dog bite become linked in the brain’s information storage. Our brains can even remember this linkage forever! But, how is the lifetime of memories adjusted? The fearful experience leaves a trace that is recorded in a small group of neurons in the brain. Every memory has its own group of neurons, composed of different types of cells. In the laboratory, we performed experiments to change the number of neurons in these groups. According to our results, in a particular part of the brain called the hippocampus, the number of neurons in a group is important for memory. Adding neurons to the group improves memory while removing them accelerates forgetting. A special type of neuron in the brain controls the size of these groups. We think that this process regulates the lifetime of memories.