There was at least one thing Zach wanted to try as a girl. She'd always been curious about how girls used the bathroom- how the mechanics of it worked, and how their parts functioned. This, then, was a golden opportunity!
"I really, really have to pee," she said, and thanks to the potion it came true. Zach could feel her little bladder full, although unlike when she was a boy, she didn't have the same sensation of tightness at the base of her penis- because, of course, she didn't have one.
Zach went into her bathroom and locked the door behind her. She lifted the lid of her toilet, pulled down her ill-fitting male boxers and shorts, and sat on the toilet seat. Being so short, her feet dangled off the floor when she sat.
"Hmm, how does this work... I've never been a girl before, do I squeeze, or..." Zach contracted various muscles, trying to release her bladder, and after a little trial and error, figured it out.
*Fsssss* went Zach's pee as it squirted into the toilet water, making a very girly sound. She could feel it leaving her body, from a little hole pointed downwards, entirely unlike her former cock.
"Ahhh... wow, this is weird... I'm peeing as a little girl... with girl parts..." Soon though, Zach was done, her little bladder empty. "Huh, I guess little girls can't hold as much." However, her bald pussy was still wet. She grabbed a wad of TP and dabbed dry, wiping from front to back. It was a weird sensation, not brushing against any dangling parts, but, she supposed, totally normal for a female. She flushed, got to her feet, and pulled up her oversized male underwear.
"This won't do... Okay, I'm wearing clothes appropriate for a little girl." Zach's clothes changed, and became a pink T-shirt, cute white boyshort panties, and a little blue skirt. "Neat! Now what should I do..." she pondered. She was a little girl, with powers to change reality at will...