Richard Feynman was once asked by a younger colleague:

"Dick, explain to me, so that I can understand it, why spin one-half [quark] particles obey Fermi-Dirac statistics."
Feynman answered: "I'll prepare a freshman lecture on it."

Feynman came back a few days later and said: "I couldn't do it. I couldn't reduce it to the freshman level. That means we don't really understand it."