Ty soon settled on an impossibility he really wanted to test. Changing sex. He had kinda wondered about what it'd be like to be a girl for quite a long time, and figured it'd be nice to try with someone he could trust not to judge him.
He focused on the idea- and when he felt a shiver run through him and his body start to change, he saw Erin's eyes widen as she witnessed the boy she'd known for four years turning into a girl. His face seemed to soften, his features softer, and the faint bulge in his neck vanished, his voice getting higher-pitched. His skin softened and most of his body hair vanished while the hair on his head lengthened, his clothes seemed to get looser on him, and under them she could see his body convulsing.
For Ty, even if he didn't see everything, he sure felt everything. He felt his chest grow soft and sensitive, his nipples perking up and soft breasts swelling under them; he felt the hormones and instincts of his body invert and feminize; he felt his reproductive system shift from male to female, as his penis shrank away and his crotch flattened and formed a vagina while his testicles became ovaries; and SHE felt her self-perception change, so she saw herself as a woman.
Once the changes finished, she noticed Erin's shocked face and, panting, said "Uh... do I look OK?"
Erin was still in disbelief that she'd seen a boy transform into a girl like this in front of her own eyes. "Y-yeah, I think it worked," she laughed nervously.
Having seen this, there was one overpowering urge in Erin's mind...