I tried a new joke in class recently; it worked gloriously.
I wrote on the board a mathematical conjecture that has yet to be proven or disproven. To emphasize that nobody knows the answer yet despite centuries of effort, I told the class, “If you figure this out, call me and call me collect,” writing my office phone number on the board.
To complete the joke, I said, “Yeah, this is crazy. So here’s my number…”
I thoroughly enjoyed my students’ coruscating groans before I could complete the punch line.