And this is why I used to joke with my tutorial partners at Oxford that the more titles someone had, the worse they were at teaching. I did find that the best tutors tended to be ones who were PhD students, with the content somewhat fresh in their minds.<p>Part of it is that when you become very senior, you start to become an administrator of a little research business, rather than a teacher.<p>Another part is that once you've internalised something, you cannot fathom not knowing it. Imagine not knowing what the complex plane is, and trying to explain it to someone who only knows what the real numbers are. Your explanations just won't make sense until you can place the real line inside the complex plane, and shepard the learner to the same state.