The Psychology Of A Joke

In his book Rhetoric, Aristotle has analyzed what a joke is: Creating an expectation and then breaking it.

“What’s the best thing about Switzerland?”
“I don’t know, but the flag’s a big plus.”

This joke builds an expectation in the first sentence (Chocolates? Watches?) but breaks it in the second, and after a confusing pause, we see that the answer does make sense: The Swiss flag has a big plus sign.