Oh man, this is better than I thought, Jon thought, as he and Karyn had sex. It was what he had always dreamed of, ever since Karyn started to mature. And when she accidentally wished for larger breasts and blond hair, his feelings towards her were reinforced. He wanted her, but he wasn't so sure she wanted him. Face it, Karyn was a tomboy. And Jon wasn't sure, but he figured that she might have been a lesbian, maybe bi. But now, with the stone, he could make her into the type of girl he longed for her to be. And one that was solely interested in him.
After they were done, Karyn collapsed into Jon's arms. She was clearly exhausted, but also completely satisfied by the experience.
Jon reached down under his bed and fingered the stone. He smiled, thinking of how to turn Karyn into his perfect lover. Then he whispered "I wish ..."
