"...back to the way you used to be."
Within moments, Jenny's upper body reemerged from her waist, and her field of vision shifted back up to her head. She glanced down at her hands and felt her face to make sure they were real. Convinced that she was back to normal, she was overcome with joy.
"Oh, thank you, Erika!" she exclaimed. "Thank you so much for turning me back! You're my best friend forever!" Out of sheer impulse she hugged Erika, until she noticed her friend seemed slightly uncomfortable. That was when she realized that although she was wearing a string bikini bottom, she was still otherwise completely naked. Slightly embarrassed, she quickly covered her bare breasts with her arms.
It was at that moment that she felt it. It was so slight that it was almost imperceptible... but it was there nonetheless.
Erika noticed her look of alarm. "Jenny? Like, what's wrong?"
"I... I don't know... it feels almost like... are my boobs getting smaller?"
It didn't make any sense. Erika had said what she was supposed to. She had said... "Oh God..." Jenny realized what was happening. "Erika... you said I'd transform back to 'the way I used to be', right?"
"Yeah..."
"I... I don't think my transformation is finished!"
"What?"
"Erika! You weren't specific enough! I'm still transforming back to 'the way I used to be'! I'm... I'm turning younger!"
It began to grow increasingly more obvious that Jenny was absolutely right. She was getting shorter. She removed her hand from her chest to find that her once-ample C-cups had dwindled to puny prepubescent A-cups. She no longer looked 17-- more like 12. Her string bikini loosened and dropped off of her just in time to reveal her pubic hair disappearing once more and leaving her completely bald and smooth down there.
"Erikaaa... Help me!! It's not stopping!"
"I don't know how to stop it!" screamed Erika, panicking just as much as Jenny.
Jenny was 9 years old before she remembered it was her turn again. "Oh! I got it!" she squeaked. Her voice was back to the high-pitched lisping little girl voice it had been before it changed. "Twanthfawwm aww dahw!"
"Dare!" shouted Erika, not missing a beat.
The years continued melting off of Jenny, who now couldn't be any older than 7. Erika, who had once been slightly shorter than her, now towered above her. "I dahw you to athk me to twanthfawwm back to a 18-yeehw-ohwd!"
A few moments passed and all that happened was that Jenny continued getting smaller. Now she was 6.
"What happened?" asked Erika.
"Nuffing happund!" shreiked little Jenny. "You wuh'nt po-thethed aww anything! I... I don't fink it'th weawwy my tuhn until... until da twanthfumayshun ith ovah!" Cute little Jenny was only 4-years-old now, and she was beginning to show signs of mentally devolving; she burst into tears and cried "I want my mommy!!". And Erika was absolutely powerless to help. Jenny's last assertion had been entirely correct. Her "turn" didn't officially start until her transformation was over, which, in this position, presented one hell of a catch-22.
At age 3, Jenny finally lost balance and toppled over onto her soft round little butt. Her bawling became less and less coherent until finally she was completely incapable of saying any actual words. "Waaaahhh!! ewika tuhn widdo jenny... intuh... buh-buh-guh-guhhh-buhhh!" Erika watched, frozen in horror, as her best friend turned back into a drooling, crying, helpless baby, lying on her back and thrashing her limbs in every which way. 2 years old... 18 months... 12 months... Erika could see her body getting redder and tinier. "No... stop!" she screamed, desperate to keep the regression from continuing. Jenny had been reduced to a newborn, freshly out of the womb. Erika was startled as Jenny's navel suddenly sprouted an umbilical cord attached to a placenta. It became clear to her then that the transformation wasn't stopping, not until Jenny was all the way back to the "way she used to be". Erika shreiked, "Jenny!!"
But it was in vain. Jenny's crying had stopped completely. She was now an unconscious fetus, and she was rapidly shrinking. Erika saw her friend get smaller and smaller until finally, the only thing left of her was a small pool of fluid on the floor. Erika knew right away exactly what had transpired-- somewhere in that pool, too small for her to see, there was an egg and a single sperm cell. Jenny had been reduced all the way back past birth and even conception, to the point where she was nothing but her parents' gametes. Jenny was gone... and had never existed.
Erika was absolutely traumatized. "Like, ohmygod! What am I going to do?!"