I intend to take a look at some evolutions of what is probably the most well-known joke type in the English language.
The simplest knock knock joke is demonstrated by the following exchange:
Knock knock!
Who's there?
Ivor.
Ivor who?
Ivor lost my door key!
The last line is invariably a pun based upon the first name given. In general, a proper name is given at the first response, and the expectation of a full name is exploited in the final answer. In a pure knock knock joke, the final answer should in some way explain why the joke-teller is knocking on the door in the first place.
Knock knock jokes, as the perceptive reader will have noticed, are not really very funny. Luckily for their continued survival, they are ripe for parody and subversion. The simplest technique is the non sequitur, or the surreal knock knock joke:
Knock knock!
Who's there?
Tom
Tom who?
George Bush!
The second response has nothing to do with the first, and is not a pun. Since this answer has some novelty value, it stands a better chance of raising a laugh than the standard issue kk joke. The surreal kk joke can be subtler:
Knock knock!
Who's there?
George
George who?
George Bush!
This is not a non sequitur, but still has an unexpected and inexplicable punchline for those (too) familiar with the formula.
The next level of the kk joke involves clever use of the formula's syntax:
Knock knock!
Who's there?
Doctor.
Doctor Who?
Of course, there is no final answer, just a wry smile. The same joke works just as effectively using "The" in place of "Doctor". A common variation is based around the phonetic certainty of the victim's second question ending in an 'oo' sound:
Knock knock!
Who's there?
A wop-bop-a-loo-bop, a lop-bamb.
A wop-bop-a-loo-bop, a lop-bamb who?
There is any number of variations on this approach, though there is a general divide between phrases that are obvious set-ups (as above) and ones that are attempts to lure the unwary:
Knock knock!
Who's there?
Boo.
Boo who?
Don't cry!
The final level of kk joke serves as a humorous commentary on the structure of the joke:
Knock knock!
Who's there?
Knock knock!
Who's there?
Knock knock!
Who's there?
Knock knock!
Who's there?
Timberrrrr!
To stand any chance (however slim) of finding this funny, the victim must have prior knowledge of the knock knock joke. This is usually the first "meta" kk joke that people hear as kids. the meta-kk joke can also be extended to incorporate the kind of pun that defines the original formula:
Knock knock!
Who's there?
Uncle
Uncle who?
Knock knock!
Who's there?
Uncle
Uncle who?
Knock knock!
Who's there?
Uncle
Uncle who?
Knock knock!
Who's there?
Auntie
Auntie who?
Auntie glad I got rid of all those uncles?
The subversion of / commentary on the structure is in place, but the joke still ends with a traditional pun.