"I'm a teenage girl." Naturally, as a horny teenager Zack had a poorly hidden fascination for girls' bodies, and he figured it might be fun to try having one himself for a while.
He had a feeling nothing would happen, but a tingling sensation ran through him after he spoke the wish. And before his eyes, his body was slowly starting to change.
His shoulders slunk inwards and his upper body muscles became more petite, most noticeably his hands, and he saw his nails turn from their natural hue to having neatly applied black polish topping them.
The changes moved downwards, and his figure altered, turning from straight to curvy, and slinking inwards as his body shrank a little. His hips, however, were getting wider, and his butt was filling out.
Pulling down his uncomfortable shorts, he saw his legs turn silky smooth and somehow both larger and smaller, though his feet were now clearly more dainty, the polish that had covered his fingernails covering his toenails too.
The sensations of the change intensified in Zack's chest and crotch, and they felt... more intense. His anticipation mounted, as did the sense that he would soon be irrevocably different.
The boy's nipples turned erect as the development they had never undertaken in their old form picked up again. They felt sensitive and ached to be touched in a way they hadn't ever, and when he rubbed them he gasped in a higher-pitched, feminine voice. Newfound instinctual desires to have them pinched and pulled and sucked were emerging in his head, and slowly they and the flesh around them swelled as they developed into soft breasts.
Only a single part of him remained masculine, and it soon gave way in a pleasurable manner. Warm semen shot from the head of his penis as it slowly prepared to reshape, and he saw his phallus receding inside him slowly. His testicles pulled up inside him, and his mind was seized by emasculation. She realized she would not be a man but a woman, and as his cock became her clit and his scrotum tugged upwards and unsealed to form her vagina, she felt herself adopt her new sex comfortably.
Zara slowly came to her senses, and blushed as she realized she was stark naked on the beach, yelping and rushing to cover her chest and crotch.
To her relief, she saw a bikini top and bottom (the latter of which had been his swim trunks) on the floor, pulling them on hastily.