in order to keep the brain fit, we must learn something new, rather than simply replaying already-mastered skills. But as we’ve said, there is a second way to increase the number of neurons in the hippocampus: by extending the life of neurons already there. Studying the mice, the team found that learning how to use the other toys, balls, and tubes didn’t make new neurons, but it did cause the new neurons in the area to live longer.
