To finish myself:

The kid looked at me and said, "D' ya wanna hear a riddle?"
And I said "Sure."
And he said "Okay. What's the difference between a tuna fish, a piano, an a tube of  glue?"
I said "What?"
He said "You can tune a piano, but you can't tuna fish!"
I said "Where's the glue come in?"
Then the teacher walked back, and we acted as if we never said a word.
Since then I hadn't been able to figure out what the punch line to that joke is. Until I saw Sky's old DNL.
Sky was in the middle, Ross had to make a joke. Ross said "Adam, what's the difference between a piano, a tuna fish, and a tube of glue?". Adam said "What?" (Meanwhile, I'm like "Wait, this is like the joke that the kid told me during detention) Ross then said "You can tune a piano, but you can't piano a tuna." Then Adam, confused, said "What about the glue?" Then Ross said

"I knew you'd get STUCK on that one."

I was like "THAT'S THE FRICKIN PUNCH LINE TO THE JOKE?!?!!!"