The best way to learn something is to have to teach it.
Eugene, an old friend and a very kind person, comes to me: What's the matter? And I answered him, in Hungarian, in very simple terms, I said: I am so stupid. And now, Wigner, basically, very basically, the kindest of all men, should have contradicted me, but he didn't. He said: Yes, stupidity is a general human property. Now, you know, the remarkable thing is that, that, that he made me feel much better. That sounded like the truth: All right, I'm stupid; so is everybody else.
Every boy in the streets of Göttingen understands more about four-dimensional geometry than Einstein. Yet, in spite of that, Einstein did the work and not the mathematicians.