Feynman was a truly great teacher. He prided himself on being able to devise ways to explain even the most profound ideas to beginning students. Once, I said to him, "Dick, explain to me, so that I can understand it, why spin one-half particles obey Fermi-Dirac statistics." Sizing up his audience perfectly, Feynman said, ''I'll prepare a freshman lecture on it." But he came back a few days later to say, "I couldn't do it. I couldn't reduce it to the freshman level. That means we don't really understand it."
David L. Goodstein and Judith R. Goodstein: 'Feynman's lost lecture, The motion of planets around the sun.' In: Engineering & science 59 (1996). Vol. 3. P. 19. [link]