The one that goes:
A girl and her dad are in a terrible car accident. The girl is wheeled into surgery, and the surgeon says, "I can't operate on this girl! She's my daughter."
Remember the answer? The doctor is her mother. And everyone was shocked and embarassed that they hadn't gotten it?
A few years ago, I used this riddle on someone young, I think it was M, but she claims not to know it, so maybe not. This person didn't even get why it was a riddle, because it was so obvious that the surgeon was her mother. Progress, no?
Well, tonight M and E were doing riddles at the dinner table, and E asked me for a riddle, so I gave her that one. She didn't even hesitate.
"She has two dads!" she shouted.