Running on a treadmill is a mind-numbing pursuit, but neuroscientists have found that mice that regularly use a running wheel grow twice as many new neurons as more sedentary mice do. Can we jog our brains on a treadmill? "It certainly can't hurt," says Henriette van Praag of the Salk Institute in La Jolla, California. "But we don't know how long a mouse has to run to get this effect, if it's like a marathon or just a five kilometer."