Knock knock. Who's there? Pane

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

Knock knock. Who's there? Canoe.

Canoe who? Canoe let me in, it's freezing out here!

Knock knock. Who's there? Knock

knock. Who's there? *sigh* Never mind.

Knock knock. Who's there? Cows go.

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