Let's observe the typical structure of a joke. It consists of two "messages" or, in general, "impacts". The first one is usually used to transfer some kind of misleading information to a person. At this point the information looks like a simple truth that people have no reason to doubt. Therefore a person begins to believe this information is really true. At the moment when a person puts absolute truth in new information the second "impact" is made. It usually contains contrary information, somewhat like this: "Relax, it is just a joke".
This kind of joke is harmful to a person's mental state because each of us has his/her own "picture of reality". The information, which is in conflict with our "picture", enforces us to make certain efforts to change that "picture". This process is quite unpleasant, especially when a very important part of the "picture" needs to be changed. So, that is why jokes may be very harmful, even if done with best intentions.
At that, it doesn't matter how "good" or "bad" the first "message" (or "impact") is. Here is an example:
1st message: - Hello Mary! Did you know your husband died?
2nd message: - Relax it is just an April 1st joke.
If Mary loves her husband, then the first message is a shock for her. Even the second message cannot compensate her stress. If Mary hates her husband, then the first message may even cause her to be delighted. But the second message immediately ?turns off? her delight. Regardless the April 1st joke provokes a person to feel strong emotions which are later taken away from them.
In connection with all these considerations the only acceptable jokes are those, which we all remember from our school years. For example, placing a sticker on a person's back or singing a song whilst altering the words to parody news current news topics.
