Joke: Why was the function not scared of getting differentiated?

Joke: Knock knock!
Who's there?
John Smith Alexander Thomas Eisenhower William Joesph Harvey Jr. the third!
...
That's what I thought.

Joke: Knock knock!
Who's there?
Knock.
Knock who?
Knock knock!
Who's there?
Knock.
Knock who?
(Keep this going as long as you possibly can.)