Related:

• Knock, Knock. Who's there? Opportunity. Don't be silly - opportunity doesn't knock twice!
• Knock, Knock. Who's there? Opportunity. Don't be silly - opportunity doesn't knock twice!
  -- Norman Dale...
• Knock on wood.
• Knock, knock! Who's there? Sam and Janet. Sam and Janet who? Sam and Janet Evening...
• How many knock-knock jokes -- does it take to screw in a light bulb? Who's there?
• Knock Knock... (who's there?) Ether! (ether who?
  ) Eather Bunny... Yea! [chorus] Yeay! Stay on the Happy side, always on the happy side, Stay on the Happy side of life!...
• Knock, knock. (Pause. Caller thinks, "Who's there?
  ) Isn't that *my* question? (Pause.) Please leave a message....
• SEMEN -- knock-up drops.
• Knucklehead: "Knock, knock" Pee Wee: "Who's there?" Knucklehead
  "Little ol' lady." Pee Wee: "Liddle ol' lady who?" Knucklehead: "I didn't know you could yodel...