prank = a trick, especially one which is played on someone to make them look silly.

“A practical joke (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, confusion, or discomfort. The term "practical" refers to the fact that the joke consists of someone doing something physical, instead of a verbal or written joke.” - http://en.wikipedia.org/wiki/Prank