"Teaching is even more difficult than learning. We know that; but we rarely think about it. And why is teaching more difficult than learning? Not because the teacher must have a larger store of information, and have it always ready. Teaching is more difficult than learning because what teaching calls for is this: to let learn. The real teacher, in fact, lets nothing else be learned than—learning… The teacher is far ahead of his apprentices in this alone, that he has still far more to learn than they—he has to learn to let them learn. The teacher must be capable of being more teachable than the apprentices."
Quoted in Salvatori, Mariolina Rizzi and Patricia Donahue. The Elements (and Pleasures) of Difficulty. NY: Pearson Longman, 2005: viii.