Once your brain is up and running, you won't be stuck with the same old neurons. Your brain will keep generating some shiny new ones. Even in adulthood, brains keep churning out new neurons, either to replace old cells or to add additional firepower.

Two parts of the brain are especially fecund: the dentate gyrus (a region involved in spatial memory) and the olfactory bulb (which sits right above the nasal cavities). A cross-section of the olfactory bulb of a mouse is shown here; relatively youthful cells, born during the animal's adulthood, glow green.

Without fresh cells, the brain runs down. Ryoichiro Kageyama, Itaru Imayoshi, and their colleagues at Kyoto University in Japan found that if they destroyed a mouse's new neurons, the animal had problems retaining certain types of memories--for example, how to navigate a maze.