Skill is a cellular insulation that wraps neural circuits and that grows in response to certain signals. The more time and energy you put into the right kind of practice—the longer you stay in the Clarissa zone,
Another example: let's say you're at a party and you're struggling to remember someone's name. If someone else gives you that name, the odds of your forgetting it again are high. But if you manage to retrieve the name on your own—to fire the signal yourself, as opposed to passively receiving the information—you'll engrave it into your memory. Not because that name is somehow more important, or because your memory improved, but simply because you practiced deeper.
Deep practice is built on a paradox: struggling in certain targeted ways—operating at the edges of your ability, where you make mistakes—makes you smarter. Or to put it a slightly different way, experiences where you're forced to slow down, make errors, and correct them—as you would if you were walking up an ice-covered hill, slipping and stumbling as you go—end up making you swift and graceful without your realizing it.
Group A studied the paper for four sessions. Group B studied only once but was tested three times. A week later both groups were tested, and Group B scored 50 percent higher than Group A. They'd studied one-fourth as much yet learned far more.