Some jokes are carefully constructed. After 9/11, for instance, these were the jokes constructed to use 'been laden' or 'telly ban' in the punchline.

Some jokes originate spontaneously by misprints or language slips.

Most jokes, however, are recycled old jokes where just the names (or the ethnic group involved) have been replaced. Boring. Some jokes I have heard about a dozen different politicians.

I love the 'unrecycable' jokes that only work in one historic situation with one set of persons. If I live long enough I'll write a book full with of date jokes which have to be explained to the readers. It will be a boring book to most (the explanation in many cases has to be longer than the joke).

One of these jokes that has to be explained to youngsters is this:
When Nixon was in the White House, he kind of disregared the First Lady due to his high workload. Ms. Nixon went to a marriage counsellor and asked for his advice. He told her to surprise the president one night with new and unexpected sexy underwear. She bought a black bra which only covered one breast, put it on (wearing nothing else) and waited for the president to come from his office. Nixon coming late from work looked at her lost in thought and exclaimed: "My God, I was supposed to call Moshe today!"

Wolfgang