Sarah leaned in close and grabbed the front of the shirt that Jon still had on. "Listen to me, you asshole. If only dreams could come true, I'd wish that you'd think that you're a girl and really really believe it. Maybe then you'd know how this felt." The stone glowed in Sarah's hand. "What the ...?"
Jon looked up. Thanks to Sarah's wish, Jon now thought that he was a girl. And because she wished that he'd really really believe it, he no longer remembered living his life as Jon the boy, but rather as Jon the girl. They were all Jon's memories, but they had been changed so that they would come from a female Jon's point-of-view.
Jon looked around at where he was. "Why am I here?" he asked.
"Huh?" Sarah still wasn't quite sure what had just happened.
"This is the wrong locker room. I need to get ready for swimming class." He ran down the hall, towards the girl's locker room. And from a distance, Sarah watched Jon walk in.
Sarah wasn't sure how it happened, but she thought that her wish had come true. Now Jon thought that he was a girl.
She smiled, walking down the hall so that she could watch Jon get thrown out of the girl's locker room. Because that's what would happen, you know.