"A decade ago, James Lang banned cell phones in his classroom. Frustrated by how easily they could sidetrack his students, Lang sought out a distraction-free environment, hoping it would help his students pay attention to his lessons. But after just a fewyears, Lang gave in. Not only was his no-cellphones policy ineffective (even his best students ignored it), he realized that he, like many of his fellow teachers, was missing an important point. The...