Swirly Twirly Hypno Coin

The Swirly Twirly Hypno Coin (also referred to as the Hypnotic Whirly Disc and the Hypno-Coin) is a novelty toy and one of the products at Krusty's House of Novelties.

Description
Krusty's House of Novelties advertises the coin under the title "Hypnotic Whirly Disc". The ad copy reads as follows:
 * Fascinating coin-sized invention. Hold plastic disc in front of person. Say "You are getting sleepy... sleepy... sleepy." They are now hypnotized. Make them do your bidding. Make them dance the Charleston in their underpants. Get several hypno discs and create large zombie army. Fun! Doesn't really work!

The coin costs 85¢ and has the order number 5556.

History
There once was a version of the Swirly Twirly Hypno Coin that actually worked, but it was banned. Krusty's House of Novelties advertised the toy version of the coin, with a disclaimer saying specifically that the coin didn't really work. In spite of the disclaimer, one working coin was accidentally shipped. The purchaser was Bart Simpson, who used the coin to prank the teachers at Springfield Elementary School. He hypnotized them into acting like chickens and left a post-hypnotic suggestion that they would resume acting like chickens if they heard the word "barnyard". Bart was never caught for this prank, but he lost the coin on the playground, where Martin found it and turned it in to Principal Skinner. Skinner in turn locked the coin in his office safe.

Later, as his supervillain alter-ego The Penalizer, Skinner used the Hypno-Coin in his campaign of punishing perpetrators of crimes against society. When The Penalizer caught offenders, he used the Hypno-Coin to make them inflict ironic punishments on themselves: for example, forcing loiterer Jimbo Jones to stand with his nose against a wall for twelve hours and making Barney Gumble do twenty thousand jumping jacks for belching in public.

Neither The Penalizer's true identity nor his use of the Hypno-Coin were known at the time, but when Bart saw news coverage of the Penalizer on Channel 6 News, he realized who The Penalizer was and how he was carrying out his punishments. Bart, as Bartman, set a trap to catch The Penalizer. When the plan succeeded, Bartman used the Hypno-Coin to erase all of The Penalizer's memories of his double identity and remember only that he was Principal Skinner. He then stored the Hypno-Coin in the Bartcave to prevent it from doing any further harm, but admitted to succumbing to temptation to pull one last prank with the Hypno-Coin. He gave Skinner a post-hypnotic suggestion to go to the teachers' lounge and say, "It looks like a barnyard in here!", thus triggering the teachers to act like chickens again.