Knock knock. Who's there? Etch.

Etch who? Bless you!

Knock knock. Who's there? Continue.

Continue who? Continue the joke. That's the whole thing. No pun.

Knock knock. Who's there? Amos.

Amos who? Amos *astounding* fact about puns is that Amos *clever* one is Amos *likely* to make you *a-moan*!

Knock knock. Who's there? Alpaca.

Alpaca who? Alpaca the suitcase, you load the car!