More out of instinct than anything else, both Justin and Ms. Heather looked for places to hide. It wasn't easy with their new, massive bodies, but they both found spots that would cover most of their huge bulk. The door opened and in walked Justin's science partner Amanda. She had come to check on the muscle growth formula that they had both been working on. She was excited to finish it, and at the prospect of becoming a huge, musclebound amazon. For so long she had felt insecure in her slight frame, and wanted to be large and powerful. Able to command respect from just her appearance, and have the physical might to back it up.
Walking over to the workstation, she noticed that the red formula was substantially more empty than it should have been. She started to grow nervous. Had someone stolen her and Justin's research? Just at that moment though, there was a loud noise, and the desk at the front of the room where Ms. Heather usually sat as she taught was flung across the room. Looking over, Amanda saw the most massive woman she had ever seen. Her muscles were throbbing. Her shoulders looked like pumpkins, huge and striated. Her biceps looked twice as big as Amanda's head. Her thighs looked like barrels attached to her round, defined glutes. Her back was an intricate web of definition with muscles bulging and swelling at the slightest twitch. All this muscle mass, and it looked like her muscles were still growing. Looking closer Amanda could she that it was, "Ms. Heather!?!"
With her hiding spot ruined, Ms. Heather stood up, muscles bulging everywhere, and gave a sheepish smile to Amanda. Suddenly though, she moaned loudly and almost fell over. Then every muscle on her body swelled up to new heights. Her abs, already impressive, became reminiscent of a brick wall. Her traps looked like they were trying to swallow her head. And her legs, already fighting each other for space, pushed even harder into one another.
"Hello Amanda, as you can see your muscle growth formula works wonders." Ms. Heather said.
"I can see this, but why did you take it? And where is Justin?"
At this, Justin walked out from his hiding place in the science closet. He was also massive. And naked. Seeing his sculpted physique and hard cock was making Amanda very aroused. But he was still nowhere close to the behemoth that Ms. Heather had become.
"Ms. Heather wanted to help us test our muscle growth formula," Justin explained. "And well, she had me try it as well."
"Still, that doesn't explain why she's so much bigger than you," Amanda responded.
"That's a side effect of the formula in men. Our uh... cum makes women grow when they come in contact with it."
Amanda was confused for a second, but a look of realization quickly showed on her face. "You two had sex!?"
"Uh... yes, we did." Ms. Heather chimed in, her growth seemingly stopped.
Amanda didn't know what to think. She had secretly liked Justin, and was feeling jealous of both his and Ms. Heather's huge muscles. But at the same time, seeing these two muscled up beauties was making her very horny, and knowing that Justin's cum could make her grow only added to how much she wanted him inside her.