"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."  – Ref: PP 52 in Feynman's Lost Lecture: The Motions of Planets Around the Sun By David L Goodstein, Judith R Goodstein, Random House, 2014.

Excerpt of the book and the quote above is also available at Caltech CODA:
https://resolver.caltech.edu/CaltechES:59.3.Goodstein