"To say that you have taught when students haven't learned is to say you have sold when no one has bought. But how can you know that students have learned without spending hours correcting tests and papers? . . . check students understanding while you are teaching (not at 10 o'clock at night when you're correcting papers) so you don't move on with unlearned material that can accumulate like a snowball and eventually engulf the student in confusion and despair."