Knock Knock!

 Who's There?

I have some sad news: Knock knock jokes aren't period. 

I had thought that they were popular in the late middle ages, but other than Shakespeare using something like a knock knock jokes in the Scottish Play called Macbeth, there's nothing. 

Knock, knock! Who's there, in the other devil's
name? Faith, here's an equivocator, that could
swear in both the scales against either scale;
who committed treason enough for God's sake,
yet could not equivocate to heaven: O, come
in, equivocator. [Act 2, Scene 3]

Actual knock knock jokes started in the early 1900s with newspaper cartoons in the format of:

Do you know {person's name}
[Person's name} who?
{Pun based on person's name}

These jokes turned into actual knock knock jokes in the 1930s with ad campaigns.

Knock! Knock!
Who's there?
Don.
Don who?
Don forget to do your shopping at the Cash and Carry

Knock, knock.
Who's there?
Rufus.
Rufus who?
Rufus the most important part of the house!

According to NPR:

"You can't turn the radio on anymore without getting one of the Knock-Knock gags," Jean Mackenzie observed in a radio-listening column in the July 25, 1936, News Herald of Franklin, Pa. "They're fun and when some of the better orchestras perform them, they're screams. But you've probably found that out for yourself."

I am disappointed, I was planning a lengthy research paper on the medieval origins, but now I have to think of something else.

Weeks, Linton. “The Secret History Of Knock-Knock Jokes.” NPR, 3 Mar. 2015, Https://www.npr.org/sections/npr-history-dept/2015/03/03/389865887/the-secret-history-of-knock-knock-jokes.