For the most part jokes are used in a pleasant way, to lighten the mood, or to of course entertain but some jokes aren't funny to everyone especially jokes that are only funny at the expense of another. Practical means reasonable or able to apply to a theory, a joke is an action or an event that causes amusement or laughter, but together these combine to make a practical joke which is a trick played upon a person often at the expense of his dignity.

Most practical jokes that are made by experienced or resourceful people are usually the funniest because of either they do not hurt the person or embarrass the person to an extreme extent, or they are so opposite of that they are extremely funny. So of course there is a line that most definitely should not be crossed...