The brain’s plasticity—its ability to change by constantly rewiring itself in response to new learning—is well established. But now scientists are finding that the brain may change even more than they thought. The general premise was that once neurons learned to represent something new—say, the smell of an apple—those neurons would remain roughly the same. But according to recent research, they can actually “drift,” so that over time, the same smell becomes represent by an entirely different set of neurons. Why does this happen?