"Hmm... I'm not sure which one! What do you think hun?" said Carrie with a huge smile as she looked over the balloons.
"I'm not sure, maybe we should think this over..." said John nervously.
"Nonsense, there's a lot of fun to be had with balloons!" said the vendor proudly. "Want to see a cool trick?"
"Oh yes!" giggle Carrie, jumping up and down.
"Alright, here you go!" He handed her a regular green balloon. "Try blowing this one up, as big as you can. Just watch what happens". He turned and winked at John.
"Okay!" Carrie grabbed the balloon, took a deep breath, and blew into it, inflating it into a respectably large balloon. "Like this?"
The balloon vendor shook his head. "Please, you can do better than that".
Carrie frowned and took another breath before blowing up the balloon, making it swell up to basketball size. She looked to the vendor, who shook his head again.
So Carrie blew into the balloon again, and again, and it swelled up larger and larger, past beach ball size. Catching her breath, she looked at the vendor. "Enough? It's bigger than me!"
"I think one last good blow should do it", said the vendor with an impish smirk. Carrie took a huge breath and blew as hard as she could...
... and that's when the vendor suddenly grabbed the balloon and squeezed it.
"Mmpff?" Carrie's eyes widened as the air in the balloon reversed direction and flowed back into her. First it filled her belly, making it swell up and push out from under her shirt, bulging until she looked pregnant. Then it started filling the rest of her body. Her arms and legs bulged, then stiffened, standing out straight from her body. Her breasts inflated like balloons in turn. First John could hear Carrie's bra snapping under her shirt, then her shirt ripped and fell off her tatters. Her hips, thighs, and ass followed, shredding her pants. Carrie's ballooning belly grew larger and larger until her entire body was spherical, her head, arms, and legs sticking comically out of it with only an impossibly-stretched pair of panties preserving some amount of modesty.
Only when Carrie had inflated to the size of a car did the deflated balloon finally drop out of her lips, and she looked sheepishly at John, a big smile on her face.
The balloon vendor grinned. "Works every time".
"Holy shit", said John at last. "My wife's a balloon?!"