Knock knock. Who's there? The setup.

The setup who? The part of the knock-knock joke that comes before the punchline, obviously.

Knock knock. Who's there? Blink.

Blink who? Blink and you'll miss this utterly tear-able pun about eye-popping humor, so prepare your ocular orb for an optic shock!

Knock knock. Who's there? Silence.

Knock knock. Who's there? Implication.

Implication who? I thought that was implied.