Knock knock. Who's there? Silence.

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!

Knock knock. Who's there? No one.

No one who? Exactly. There's literally nobody here. I just wanted to see if the door was locked.

Knock knock. Who's there? Cows go

Cows go who? No, cows go 'moo'