Clutching the stone (which was actually, unknown to him, one of the Twin Stones), Jon said "I wish that Sarah was bisexual." The stone glowed granting his wish.
At that same moment, the stone in Sarah's pocket also began to glow, as she turned bisexual. Unfortunately, that didn't make her want to join Greg and Jon anymore than before. Now, if Jon was a girl, perhaps she would.
Unknown to most people, Sarah McMillan was bisexual and was very attracted to the female form. More than anything, she loved girls' breasts, how they jiggled and bounced. It was mesmerizing. Unknown to Sarah, however, the only reason she thought that she was bisexual was because Jon just wished for it less than a minute ago. Since she didn't hear the wish, her memories had changed to fit the new reality.
Also at that moment, someone else in the high school was changed. Because these were the Twin Stones, the wish was granted to both Jon and Sarah. But Sarah wouldn't refer to herself in the third person, so if she had wished that same wish, she would have been referring to the only other Sarah at her school. Yes, it was very odd, considering how popular a name like Sarah was, but there were only two Sarahs in the entire school, teachers included. And the other Sarah was none other than Sarah Taylor, one half of the Taylor Twins. (The other twin was Jennifer Taylor.) Both of the twins looked exactly alike, but they had a few differences in personality. Jennifer was outgoing, into acting, and basically a party girl. Her sister, on the other hand, was studious, stern, and a little nerdy but not enough to place her in that crowd. She had aspirations of being a businesswoman some day, seeing herself as the CEO of some major corporation. But there was one thing that even her sister didn't know about ... she was bisexual.
Thanks to Sarah McMillan's unspoken wish (but mostly thanks to Jon, since he was the one who wished it), Sarah Taylor now thought that she was supposed to be bisexual and had been for a while.
