George is a great puzzler, so I was extremely surprised when he didn't immediately know the answer to a really famous puzzle. It's a puzzle that you probably did years ago, and have heard so often you can do it from memory rather than working it out. It's also not really that difficult, so I was also surprised when it appeared to be stumping him.
"Come on, surely you know this one." I said.
"I don't. And don't call me Shirley." He answered grumpily. I could tell his mood was declining rapidly, but like any great puzzler he was down and not out, and I watched his facial expression change as he reached into his mental bag of tricks. He nodded towards a conveniently located white board. "Have you got a marker for that?"
I handed him one, and he drew up the following diagram:
He stepped back, admiring his work, beaming proudly. "Well, now the solution is very obvious!" he commented.
And indeed it was.
The question for you is:
what was the puzzle?