Sam put on his sister's biini bottom and looked at himslef in the mirror. He decided that this will do for him to go out swimming.
He left his locker and went to the beach. He felt a bit uncomfortable as he was aware that his swimwear looked a little bit feminine. But noone seemed to notice and after some minutes he didn't think of the bikini bottom any longer, He hafd a good swim in the ocean and returned to his towel for a sunbath. He closed his eyes amd was soon fast asleep. Therefore he failed to notice that something odd started,
When he laid down on his towel a bulge was visible in his "trunks". Slowly this bulge shrink until it was totally gone. Instead of his penis and testicles he had a vagina now between his legs.
As Sam finally woke up he was unaware of the alterations that happened in his crotch. He felt an urge to pee and went of to the toilet. There he tried to fish his member out of his "trunks" and failed to find it. He lowered the bikini bottom to look more carefully and was greated with the sight of a vagina instead of his dick.
He rushed to the net mirror to find that his legs and feet also had changed into a more feminine form. The upper part of his body remained unchanged.