Emily's begun to grow

Emily looks down to see her arms are already thick as posts, bigger than anybody else in school’s. Even bigger than the adults. She was practically a bodybuilder already, at least in her arms.

“I… I… wow! How can this be happening already!?”
“I… don’t know,” Amy stammered. “But isn’t it great!?”
“My god, yea! You have no idea how this feels!” Emily gushed, flexing her beefy arm.
“I don’t,” Amy drooled. “But I want to! Lemee try!”

Amy took her turn trying out various machines, finding they all worked just as well. She was bulking up a bit everywhere, soon sculpted like a goddess for a girl her age. Full grown women would have murdered for a taut figure like hers. She looked like a shorter version of a professional bodybuilder.

Amy was posing excitedly in front of a mirror when Emily looked down at herself again. Her arms had shrunk back to normal, gradually. “Crap. I’m back to normal again.”
“Yea, that’s a shame,” giggled Amy, poking Emily in the stomach and letting it wobble a bit. “We can’t all be fitness models, though, Em.”

“Maybe we all want to,” murmured Emily, looking towards the floor shyly.