Every joke is like a mental exercise. In advanced cultures, a joke is used to keep people sharp, whether they know it or not. People who are slow are made fun of, since they are unable to know when someone is being serious or trying to mess with their mind. This is particularly common in Ireland and Irish New England. It is sometimes hard to interpret people who are accustomed to this kind of humor, and these kinds of jokers struggle to avoid being understood. Hence the rumor that Freud said the Irish are impervious to psychoanalysis.