Ty's mind was drawn to the idea of what it'd be like to be a woman. Since he'd been a little kid, and especially since he'd hit puberty, he'd had a weird curiosity about how as a boy, he got treated and was expected to like things one way, while girls were treated and expected to like things another way. Especially with how, as a sheltered teen, he felt pretty distant from girls even though he often liked them. He wondered what it'd be like if he were a girl, what SHE'D be like if she were a girl.
He felt a weird warmth spread through his body, slowly understanding that he had begun to change from male to female. A prickling, tingling sensation spread through his skin, which softened and smoothed rapidly, and he pulled his clothes off, stood naked in front of his mirror as he became she.
The first really visible signs of this were how Ty's muscle structure reshaped itself. His arm and leg muscles shrank and turned more lithe, and his hands and feet got more petite. His shoulders narrowed and his hips flared outwards, giving him a hint of an hourglass figure. The changes shot up his neck, which narrowed while his face was resculpting into a more feminine shape, with softer cheeks and puffier lips.
Next the changes seized upon the hint of femininity on the boy's chest. He gasped as he felt his nipples grow sensitive and erect, and with the instinctual enthusiasm with which he had discovered the erogenous zone between his legs a couple of years prior, he moved to rub at them, his head soon consumed by the pleasure. It only intensified as they swelled and widened, and soon Ty had begun to sprout breasts. He cupped his soft mammaries and whimpered with pleasure.
He got shaken out of his aroused state when he noticed his hard penis and testicles between his legs, the only thing denoting Ty as 'he' at all. He braced himself for pain as it began to recede- only for the pleasured feelings to get more intense. His shaft gradually shrank into his body and its nerve endings bound up tighter and tighter around the tip as it slowly formed into her clitoris. His testicles slid up inside him and linked up to a womb, his scrotum becoming flush with his crotch and unsealing itself to form her vagina.
Ty breathed deeply as she finished changing, and stared at herself in amazement. She had changed from a young man to a young woman without any hint of his previous sex. It surprised her how in a lot of ways, she looked like the boy she had been- she was still kinda scrawny as a girl. But she still felt intrigued by how she had changed.