Because Sarah made them stay together for one minute, their sexual drives were heightened by twenty times. And every time they tried to not think about the wishing stone, they ended up thinking about it anyway, so that increased their sexual drives as well.
Only about halfway through the minute was when Karyn and Jon both started thinking about fucking each other, and that's when their transformations began.
By the time the minute was over, they were well on their way to becoming what Sarah had wished them to be. Jon: a busty, blonde, vapid, air-headed bimbo. And Karyn: a scrawny 105-pound nerd.
