Knock knock. Who's there? Pane

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

Knock knock. Who's there? No one.

No one who? Exactly.

Knock knock. Who's there? Wu

Wu who? Wu don't know, I'm having an identity crisis

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.