Most jokes follow rigid principles, if you care to analyze them. A favorite of mine, as joke principles go, is the "divide-by-zero"-joke, or "multiply-by-zero"-joke, as the type may just as well be called - the mathematical analogy is equally out of place in both cases. However, as the common principle in these jokes is a play with nonexistence, the term "zero" in itself is not entirely out of place.
Customer to waiter: "A cup of coffee, without cream, please!"
The waiter runs off to the kitchen and returns: "The cook is unfortunately out of cream. Would a cup of coffee without milk be OK?"
Bystander to angler: "How many fish have you caught today?"
Angler: "Well, if I get the one I'm waiting for, and two more, then it will be a total of three."
The following was a favorite in the old Soviet Union, with its chronic shortages and queues:
Customer: "I'd like to buy a jacket, please."
Shop attendant: "Sorry Comrade, this is the wrong shop. This is not the shop that doesn't have any jackets - this is the shop that doesn't have any trousers. The shop that doesn't have any jackets is across the street."
Don't be deceived by the apparent innocuousness of the examples above - there is a delightful aura of abstract evil radiating from "divide-by-zero"-jokes, an aura that sex-jokes, confused-reference-jokes, multiple-reference-jokes, racist jokes, fart-jokes etc., decidedly lack.