I don't know if the joke from my native language gets through, but it is not unlike the French Jean archetype-series:

"This guy came to me and asked me to explain it. I explained him once, and he didn't understand. Then I explained him again for a second time, and he still didn't get it. By the third time I was explaining it to him, even I managed to understand it, but he was still clueless."

It is why experienced teachers are good...