“Once I asked him to explain to me, so that I could understand it, why spin-1/2 particles obey Fermi-Dirac statistics. Gauging his audience perfectly, he said, “I’ll prepare a freshman lecture on it.” But a few days later he came to me and said: “You know, I couldn’t do it. I couldn’t reduce it to the freshman level. That means we really don’t understand it.”
— “Richard P. Feynman, Teacher,” David L. Goodstein, Physics Today, vol. 42, no. 2, 1989, pp. 70-75.