Athena took the stone from Zoe, not realizing that it was actually The Random Stone, and said "I wish those two girls were the fattest girls in school." As she said it, she pointed right at Melissa and Jessica. But since this was the Random Stone, the wish she made wouldn't be granted to her, but instead, someone within fifty feet of her.
It would be as if this other randomly-picked person had made the wish themselves (so whichever two girls they were looking at would suddenly become the fattest girls in school). Although, since the person didn't hear the wish, their memories got changed along with everyone else's (except for Athena and Zoe's, of course).
It turned out that the person the stone picked was ...
