A small machine made of a rubber band, wooden sticks and a spool. The rubber band is put through the spool and the sticks are wound tightly on each end, creating tension. The user then sits on a wooden or metal chair with the fart harp under his butt. At the proper moment the "harper" leans to one side, allowing the fart harp to spin against the hard chair. This creates a loud farting noise.
He was thrown out of class for sounding a "fart harp" in the middle of the lecture.