"Ok, I'll help you, but you need to tell me how this thing works first," said Judy.
"It's simple," said Jim, "You have to say what you want to link this balloon to and inflate to increase it or deflate to decrease it. You can stop the link by saying 'release'. So all you need to do is say 'control Jim's size' and blow it up until I'm back to normal height then say 'release'."
"Ok, sounds easy," said Judy before she picked up the balloon. She said, "Control Jim's size" before blowing into it. Jim gave a sigh of relief as he felt himself growing. Pretty soon, he was already 12 inches tall, which put him out of immediate danger of being crushed by accident, but he still had more to go. But as he grew larger, he became more aware that he was naked in front of Judy and used his hands to cover his crotch.
But just as Jim was eye-level with Judy's belly button, a thoughtful look entered her face before she took the balloon from her mouth and said "Release." She let the air go out, but Jim didn't shrink anymore.
"Hey! What are you doing?" demanded Jim, "I'm still too short!"
"This balloon can increase anything about a person?" asked Judy.
"Yes, and decrease it too. Now keep growing me," said Jim.
"What else have you used it on?" asked Judy.
"If you really have to know, I used it to grow my feet for a while, I made a girl's boobs grow, I made a girl lose weight, and I played a bit with a baby's age," said Jim, his patience starting to shorten.
"So it can change someone's age?" asked Judy eagerly.
"Yes, yes, you know what, I'm big enough to inflate it myself now," said Jim, "Give me the balloon, Judy."
"Not yet, Jim," said Judy, "I wanna see how it works for me." Seeing how Jim apparently shrank out of his clothes, Judy started removing hers. Since Jim was still so short, it was easy to keep him back while she stripped and hold the balloon out of reach. She got embarrassed when she was naked, but she more focused on the balloon. "Control Judy's age," she said before she started blowing into it.
The effects were quick to be noticed. Judy got taller, the remaining baby fat on her body was starting to drain out. Not that she had a lot left considering how much soccer practice she had. Before long, she had gone from 9 to Jim's age of 12. As she passed that, puberty began. Her hips began to expand to the sides and become more rounded. Her face began to thin out a little, becoming less round. Her blonde hair also began to grow longer, reaching down to the growing cones on her chest.
As the balloon inflated more, so did her breasts and ass. Her breasts were beginning to round out as her nipples grew a little. Her ass was growing out to match the expansion of her hips. Her pussy was well over Jim's head now and he could see a blonde bush growing over it. Her extending legs were also becoming curvier, but there was a noticeable amount of muscle, presumably from her soccer playing. Her arms were getting a bit of tone too and the hands that held the balloon had longer, daintier fingers with longer fingernails. The balloon was obscuring her face by this point, but it was a lot more beautiful with full, pouty lips, long eyelashes, a slender nose, and a slightly pointed chin. Her eyes had changed a bit as well as mature knowledge was filling her brain.
When Judy reached 18, she removed the balloon from her luscious lips and said, "Release." She let the balloon deflate and noticed that she was taller than before. Ignoring Jim for the moment, she walked over to his mirror to look at herself. Judy saw a pretty young woman with long blonde hair. She grinned and posed in front of the mirror, trying to look as sexy as she could. She cupped her new breasts, which her expanded knowledge told her were BB cups. "Hmm, Mama likes," she said happily, her voice sounding a grown woman's instead of a little girl. Judy turned and bent over, looking back at her ass. It wasn't a fully bubble butt, but it was cute and tight. She gripped her hips and said, "This is gonna get me some attention."
Jim was really annoyed with Judy, particularly with leaving him in such a diminutive state. But even more to his chagrin was that Judy had grown up to be pretty hot. His hands were still covering his crotch, but now it was to hide his hardon. "Ok, you're grown up now. Now make me grow up," demanded Jim.
Judy looked over at Jim and said, "Oh, you want to grow up, 'little' brother?" she asked, putting extra emphasis on 'little'. She wasn't just taller than Jim now, she was older. She held up the balloon and gave a smile that bordered on condescending before she said...