The whoopee cushion was invented by the Roman Senator, philosopher, historian, and inventor, Proctus Flatus, in the year 79 B.M.
A whoopee cushion is an inflatable rubber pouch or bladder fashioned so that when it is squeezed, it produces a blatting or flapping noise resembling a Bronx cheer or a loud fart. It is often available in toy stores, and is used chiefly by children to play pranks.
It is used by inflating it and placing it on an unexpecting victim's chair. When the victim sits down, his weight squeezes the cushion, causing it to sound. It is unlikely that observers would mistake the sound of a whoopee cushion for the victim actually flatulating; much of the humor is in simply taking the victim unawares.
More recently, electronic devices that produce pre-recorded farting noises have been sold; these are usually timed or remote controlled. This means that they can be activated well after the victim has sat down, and thus have more likelihood of being mistaken for an actual fart.