Here, have a lightbulb joke.
“How many Vietnam vets does it take to replace a lightbulb?”
“Uh… I don’t get.”
“You don’t get it, eh? Well, YOU WEREN’T THERE, MAN!!!”
Saving this one in case a regular joke bombs and nobody laughs. When nobody laughs, I say “Jesus, I didn’t think that was too high-concept. Do you want knock-knock jokes? Because not laughing at my good stuff is how you get knock-knock jokes!” Then I pause for a second, brighten up, and tell this joke. No plan on what to do if it fails to get a laugh as well. Incidentally, I’ve seen this joke in various forms on the web, so I’m not going to claim it as original. What probably happened is that I read something like it a long time ago, forgot it completely, and it came to mind while trying to brainstorming new material. The only thing I think I’ve added there is the implicit awkward pause after the joke-teller says “Four”. Even that detail is probably not new.