Did you ever make up a joke? I mean entirely, start to finish, not using anybody else's ideas or repeating something you've heard before? Not just filling in the blanks, as in the "knock knock" variety, or retelling a funny story?
Did You Yourself Ever Make Up A Joke?
Once I put those restrictions on you, your first answer of "yes" probably became "well, not really." Or, if you still said "yes," I bet you only made up one, a few years ago.
That's interesting, because there are literally thousands of jokes floating around, all the time. The internet is full of jokes. Go to a party any weekend and you might hear several new jokes.
Where Are They All Coming From?
Some people, like comedians, actually sit down and invent jokes. But the neat thing is that most jokes come about without anyone inventing them.
Philosopher of science Daniel Dennett uses jokes as an example of how evolution works. Let's say the President does something embarrassing. Someone comments on it in a funny way.
Someone else repeats the comment with an embellishment to make it funnier. Someone else turns it into "What does the President say when...?" And so on. Jokes evolve.
What's The Punchline?
This is why you run across so many jokes you've heard before, though you may not know it until the punchline.
They've gone through so many changes you may not recognize them at all. Then suddenly you say "Oh, I've heard this." What you actually heard was a kind of joke ancestor.