"Pay attention to your students. Hear what they say, try to find out what their capacities are, what make sense to them. Adapt what you are doing and saying to those capacities, but make your students stretch upward. I think the trick is to adapt to the level of a student, but never rest on that level — always make them reach out. … If a student does not quite get it the first time, he or she will come back and get it later. If you don’t set your writing — and teaching — at a level that makes them stretch, they are never going to develop their intellectual muscle."