K is for "Knock Knock..."

Over the winter holidays, my sister and I wrote a series of Serious Knock-Knock jokes. They were along the lines:

Knock knock!
Who's there?
The police. There's been a complaint.

We had pages and pages of these, probably because it was very late at night and we were giddy.

Recently, I read in a school paper someone else's string of slightly out-of-the-box Knock Knock jokes. It's a lost artform, it really is. What are some of the best ones you've heard?

G'night!
Tome