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. His conduct, therefore, often produces the impression that we properly learn nothing from him, if by "learning" we now suddenly understand merely the procurement of useful information. The teacher is 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.
-- Martin Heidegger*
* What is Called Thinking? (Translated by J. Glenn Gray). One of my guiding concepts for teaching. Sometimes, I even share this "secret" with my students.