If you move enough, your muscles change and grow. So does your mind. The brain initiates movement. But it is, in its turn, remade by movement. New cells are born; new vessels sprout. The same process operates body-­wide. No cell in your body is unaffected by motion. Your very DNA is changed.

There was no girls' cross-country team at our high school, since cross-country courses were two or three miles long, and, at that distance, a girl's uterus could fall out.

There is no medicine or other intervention that appears to be nearly as effective as exercise in maintaining or even bumping up a person's cognitive abilities.