The Practical joke reference article from the English Wikipedia on 24-Apr-2004
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Practical joke

A practical joke or prank is a humorous practice (usually in action, not just in words) in which another person is fooled, annoyed, or embarrassed in what the perpetrator imagines to be a mild and light-hearted fashion. Some types of practical jokes include:

Practical jokes are features of various kinds of holidays, such as April Fool's Day, Halloween, and in Spanish-speaking cultures, the Day of the Holy Innocents. They also feature in various rites of passage, such as stag nights.

see also: joke, prank, humour, sadism, laughter, hack (MIT student pranks)