Gag gift

A practical joke device is a manufactured prop or toy intended to confuse, frighten, or amuse individuals as a prank. Often, these objects are harmless facsimiles of disgusting or terrifying objects, such as vomit or spilled nail polish. In other instances, they are created as seemingly harmless items designed to humorously malfunction in such a way as to confuse or harm the target of a prank. The devices are frequently sold in magic or specialty shops, purchased over the Internet, or crafted for oneself. Perhaps the most notable such device is the Whoopee cushion.

Though commonly employed at events and gatherings, practical joke devices are sometimes seen in everyday life, either as a mechanism of play by children, or among adult co-workers in a work environment. In addition to commercially manufactured practical joke devices, everyday objects have been converted into joke devices by purveyors of pranks.

Historical accounts of practical joke devices date as far back as Xenophanes (570 - 480 BC), who wrote of such gags.

Artificial body parts can be, for example, attached on or under autos (to pretend as if someone's lost a limb after they're run over).

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Wikipedia