http://en.wikipedia.org/wiki/Practical_joke
A practical joke[1] (also known as a prank, gag, jape or shenanigan) is a mischievous trick or joke played on someone, typically causing the victim to experience embarrassment, indignity, or discomfort. Practical jokes differ from confidence tricks or hoaxes in that the victim finds out, or is let in on the joke, rather than being fooled into handing over money or other valuables. Practical jokes or pranks are generally lighthearted, reversible and non-permanent, and aim to make the victim feel foolish or victimised to a degree