Knock knock. Who's there? The sound of one hand clapping.

The sound of one hand clapping who? Exactly! You're not supposed to be able to answer that.

Knock knock. Who's there? Sewers.

Sewers who? Sewers *nice* of you to ask, but these puns are really *sewers*-ing my patience, making me think it *sewers*ly time to *sewers*-t out a new joke writer!

Knock knock. Who's there? This is a knock-knock joke.

This is a knock-knock joke who? Just trying to figure out where we go from here.

Knock knock. Who's there? Lettuce.

Lettuce who? Lettuce in, and we'll 'romaine' here all night, serving up 'head'-spinning puns until you 'leaf' feeling 'crisp' with laughter!