Stephanie Coontz: One of the most important reasons why we go to school is to build up this thing called cognitive endurance. She says in one condition, kids were asked to do something which basically was like training. When you get something right, she says, it levels you up. It's almost like you go to your physical trainer and as soon as you can lift five pounds, they move you up to eight pounds,. So it's getting harder as you get better, to maintain this cond of load.