Oh my goodness, I was making up these knock knock jokes to piss off my 4-year-old cousin because they kept going over her head. I had a whole bunch, but these are the two I remember.

Knock knock.
Who's there?
Doctor.
Doctor Who?
You really should watch that show, it's great!

Knock knock.
Who's there?
Knock knock.
Who's there?
Knock knock.
Who's there?
Sorry, UPS tried to get ahold of you three times; You'll have to come to the office to pick up your package.