Max had been considering something since his father had died. He felt pretty out of sorts with the rest of his family considering his status as the only boy, and didn't even feel that attached to his gender. That might be an interesting place to start, especially because it would test the limits of the "What If" machine.
Cautiously, he typed "WHAT IF I WAS A GIRL?". A blinding flash filled the room, and Max found himself in some kind of void. He had a weird vision- a penis cumming inside a vagina, a mass of sperm surrounding an egg with one fertilizing it, and then the footage reversing and a different sperm moving inside the egg instead.
Max could then see his body again, first noticing that he was suddenly naked and then that he felt strange. He realized his flesh was moving about, resculpting as if he were made of clay. He saw his muscles and hands turn more petite, his skin get almost hairless, his figure slink inwards. He felt a twinge of sensitivity in his chest and his nipples grew large and puffy, and soft flesh swelled around them. Another surge of sensations emerged in his crotch and he saw his penis shrink down and his testicles pull up inside him, his scrotum splitting down the middle to form vaginal lips.
She felt herself adapt to her new sex quite fluidly, covering her chest and crotch embarrassedly- only for the light to brighten, fade and her to find herself back in her room. Thankfully, she had a T-shirt and skirt on now. Her memories of her new life were coming to her now...