The easy way

You rest quietly against the glass for what felt like an eternity of pondering but actually must have only been a few moments. "Fine, what do you need me to do?"

Stephanie sighed, presumably in relief. "Thank goodness you're on board, the sooner the better. Okay, so from what I read, I'll open up like I did earlier, you'll take off your clothes and slip your legs in first, then hands and head last. I'll close up, you get dressed again, and then... the next phase of the experiment, I guess."

"You're awfully calm about all this," you said as you started to unbutton your shirt.

"Oh, trust me, I was screaming and yelling and pounding on the glass while you got your beauty sleep. Didn't do much, every time I hit the glass my hand goes flat, and that is unnerving, so I spent some time just getting used to how it all feels. But I only know what it feels like to be hollow so far, and let me tell you, I could really use some stability even if I'm no longer, well, in control of myself. I'm putting a lot of trust in you here, Will, but it's mostly because I don't have a choice."

Once you'd discarded your shirt, next was the pants, but you stopped at the boxers as you glanced at the other occupant of the cell. "Uh... turn around?"

Steph just rubbed the bridge of her nose, as you realized she was already mostly naked herself, standing topless in front of you with her small chest, wearing only the panties. "That's what you're going to get hung up on, seriously? That I'm going to see your body? Look around, Will, nobody gets any privacy here, all the other experiments can see you, but they're all busy freaking out about their own situations, yeah?" Taking a deep breath and closing her eyes, she reaches out and grips your hand, allowing you to feel her soft skin in your palm. "Please, just let's get through this without any delays, okay?"

Reluctantly, you slipped out of your boxers next, revealing your semi-erect self to the woman occupying the same cell as yours. She glanced down for just a moment to see your form, before she turned around again without any remarks. Even if it was a bit embarrassing that you were somewhat excited by Stephanie's body, she seemed to have been expecting a reaction like that. Or at the very least, she was pushing her thoughts on the matter aside as she tried to keep you on task. "I'm opening up now, and then I'll lean down a bit. Get your legs inside first, and then let's get settled before we go any further."

"That's the weirdest thing I'd ever heard, but alright," you said.

Stephanie's back again opened, revealing her smooth skinlike interior to you once more, as she braced her hands against the glass of the container, holding onto the cylindrical wall. She bent down, somewhat crouching in a way that would allow you to step into the sizable hole in her body and push inward. You also braced against the glass, before you lifted a leg up and began to slide it into her. Surprisingly, it felt incredible, almost like Stephanie was caressing your leg the whole way down as your foot pushed through the gap in her upper leg and worked its way down to her ankle. Stephanie shuddered, perhaps even lightly moaned as she felt your intrusion, stretching her limb out as your foot's shape distorted her own body briefly. Pushing further in, you felt your foot slide into place, still hugged by Stephanie's warm interior, feeling your toes lining up with hers. Her upper leg had slightly crumpled down, folding upon itself, which made it easier for you to lift up your second leg, now supported by the one inside Steph's body, to insert it in the same way. Once again, Steph shuddered and made pleasured noises, clearly enjoying the sensation. Perhaps that was by design, so she would want to be invaded by another body? If she was going to lose her autonomy, it might as well feel nice in the process, you thought.

"Almost... there..." Steph said, her breaths quickening as she felt her body being stretched by you from within. "Gently pull up on my hips, I'll get you situated with the rest."

A thought occurred as you reached to Steph's hips, realizing that you were feeling some padding between her outer skin and the part within her that rested against you, so she wasn't simply like a pair of pants--rather, she reminded you of those silly false-muscle shirts that get used for Halloween costumes, to create the proper silhouette. Still, even if she was padded in some places, there was a couple areas that you thought might pose a problem... "What do I do with, y'know... Will Junior?"

This time when Steph sighed, it was a return to exasperation. "Of course you'd call it that... still, a valid question, I guess. Once our hips are aligned, it'll get... pushed into place, it's supposed to be an automatic thing where our anatomy aligns so all bodily functions work through both of us. Again, I don't know the details, we're going to just have to find out."

"Alright, well, here we go then..!" you gripped Steph's hips, and pulled upward, stretching her lower body up around yours until your hips did fully align. Amazingly, as you looked down during this process, the shorter, petite legs of Stephanie had grown to match your height, while the padded portions inside her fit against you in such a way that her legs flared out in a pleasant feminine shape as they continued up to the hips, something not characteristic of you and certainly not what you'd expected given Stephanie's rather slender appearance before you understood anything about her hollow insides. It seems there was just enough padding under her skin that it was meant to match your body type!

Before you could look down for much longer, you shuddered when you felt a new sensation, as your now hardened dick pushed upward into a cavity within Stephanie. As you grunted in pleasure, Stephanie howled in ecstasy at this unexpected penetration! "G-god, it's... like you're fucking me from the... inside..!"

You leaned back slightly, taking a step with these newly-shaped womanly legs that were now at your command. You could feel Steph twitch around them, trying to flex or influence movement in some way, but her efforts remained futile, with only a fraction of the strength you possessed. Still, she did have the ability to move around your cock, her muscles lightly flexing and pushing up and down, seemingly in a way to get comfortable. "Hnnn, I can't believe it," Steph said, "this feels too good! You're in me, but... like, really in me! I don't know how to describe it, but it's so sensitive..."

You held your hands on the widened hips you and Stephanie shared, pushing against them lightly as you continued to adjust your own self. Amazingly, due to the padded internal structure of Steph's body, you didn't see even the slightest hint of a bulge in her pelvis where your manhood clearly resided--though when you reached around and pushed your fingers against her panties, you could feel a hint of your hardness within. That touch only served to make Stephanie moan more, as she flexed around you, pushing against the glass to **** you in slightly deeper, before pulling lightly away from you. Was she actually trying to have sex with you like this?

"I thought you said no delays, huh?" You put your hand on the glass again, looking down as you spread your legs slightly to make it easier to push up inside what was essentially an inner vagina embedded within Stephanie. Of course, from the outside, it looked like Stephanie's newly-minted ass and hips and legs were doing the spreading, bouncing lightly as the two of you seemed to push against each other in this strange interaction.

Steph nodded. "You're... you're right. But..." you felt a tight squeeze around your whole lower body as Stephanie held herself tightly to you, although her weakened upper body seemed to dangle limply from your waist as she rest against the glass to do whatever she could to stay upright. "This feels so good right now, it's going to drive me crazy to have you inside me like this and not do something about it! Look, Will, you're in charge here. Either you keep putting me on to distract me from all of this, or you find a way to make us cum like this right now and we worry about the rest later, but don't just stand there..!"