Sit still, close your eyes, and rest your attention on your own breath. Nothing else. Within seconds the mind bolts with a worry, a memory, a song fragment, or an itch. You drag it back. It bolts again. This small, frustrating act, repeated daily, physically rebuilds the brain doing it.

A Harvard neuroscientist named Sara Lazar found the evidence by accident. A running injury sent her to physical therapy, the therapist told her to stretch, and she took up yoga, half rolling her eyes at the instructor's talk of open hearts and clearer minds. Then she noticed she was calmer, steadier, and harder to rattle. So she put meditators in a brain scanner.