Knock knock. Who's there? Silence.

Knock knock. Who's there? Pane

Pane who? I'm having a window of opportunity to let you in.

Knock knock. Who's there? Cows go.

Cows go who? No, cows go 'moo'.

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!