The first person to find the device in the Magic Shop was a boy called Max Webber. Max was 17 and intrigued by the idea of changing gender- he had been for a long time, at least since he'd started puberty if not before. He'd wondered about the possibility he might be trans or not straight, but felt too shy to experiment with it, and he figured this thing could really help with that. He hastily took the device from the shelf, bought it and rushed home.
Once in his bedroom, he booted up the device and it scanned him, showing a diagram of his body naked. Before using it on himself, he checked the instructions to assure himself that yes, this was reversible, and reading that made him even more excited. He decided to undress before using it, and set it to affect his whole body. He also realized he should probably set it so reality was altered by the change- that way, no one would be confused that he wasn't a boy anymore.
Having established all of this on the device, he clicked a button which was labelled with a combined Mars and Venus symbol, and it changed to just a Mars symbol on a blue background. This rotated clockwise and the colour behind it shifted through purple hues until the symbol's arrow sat at 6 o'clock, its points repositioned, and it became a Venus symbol on a pink background.
Once this little animation finished, Max started to feel a warm tingling all through his body. He looked up at himself in the mirror, and before his eyes it seemed to reshape itself as if his skin was being moulded by an artist. He noticed his skin felt more supple and smooth, and much of its hair had receded. His facial features softened, his lips turned puffier and his neck narrowed with his larynx receding into his throat, giving his voice a higher pitch. His shoulders slunk inwards, his hips widened and his figure curved subtly, while his arm and leg muscles became more lithe and petite while his hands and feet shrank slightly.
The tingling now turned to arousal- Max had never been so turned on in his life as the sensations started to centre around his chest and crotch. His nipples swelled to several times their normal size and felt amazingly sensitive, and when Max reached to touch them an involuntary moan escaped his lips. As his breathing grew heavier, breasts developed under his touch, and soon large soft mounds of flesh had formed on Max's chest.
He finally managed to take his hands from them, and stared in amazement at his new boobs. Something he started to consciously realise from all this was his gender was more tied into his body than he'd always assumed. Now that female hormones were surging through his system and his body was almost entirely female, the idea of being a girl was very enticing to him.
And so he watched in anticipation as at last his penis started to recede away. The shaft shrank slowly back into the body, and what remained seemed to grow more and more sensitive. But what really struck him was how he felt when his testicles moved up inside his body and reshaped into ovaries. He felt... castrated. Emasculated. Like there was no boy left now they had feminized. And she liked it. She liked the idea of being a girl, and it aroused her to see what had been a penis finish shrinking away into a clitoris and feel her scrotum divide into vaginal lips at the end of a tract reaching inside her body.
Maxine looked at her girl's body in amazement, realising it felt as natural to her as having a boy's body had a few minutes earlier, though her curiosity about experimenting with genders remained. She wondered what to do now...