Biff has always picked on Jon, but now Jon had a chance to get a bit of revenge. Sure, Jon felt a bit bad about it for a moment, but then he remembered how much Biff had picked on him. He knew exactly how to mess with Biff while still somewhat taking advantage of him.
Jon took out the wishing stone. For the next week, Biff would believe himself to be transformed into the cute and sexy cheerleader of his dreams. When Biff looks in the mirror, he'd see a cute and sexy cheerleader. He'd dress and act the part. The catch, of course, that only Biff would see himself like this. Everyone that looked at Biff would still only see Biff. If Biff was wearing a cheerleading uniform, Biff would see himself as a cute cheerleader in a uniform; everyone else would simply see Biff wearing a cheerleading uniform. If anyone tried to tell or explain to Biff that still look like Biff, he wouldn't believe them, and instead insist he was a sexy cheerleader and try to change the topic by asking to have sex. As a final kicker, if Biff asked to have sex with anyone, then that person won't be able to stop themselves from having sex with Biff.
Jon smiled...
