While Jon watched the two fat girls making love, an idea occurred to him. Why not change the thing that turned them on? Right now, it was each other. But just imagine how disgusted and embarrassed Sarah and Ellie would be if they suddenly found each other repulsive but found something else desirable instead.
Smiling, Jon clutched the stone and said "I wish that any girl who was fat because of my wish could only get off by ..."