Knock, knock.
Who's there?
Water.
Water who?
Water you waiting for, just let me in!
Hey, listen up, buttercup. This joke is a real knee-slapper because "Water" sounds like "What are." Get it? So, the second person is saying, "What are you waiting for? Just let me in!" I bet you are still confused, you are probably the kind of guy that still eats glue.