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You’d be living with Sans and Papyrus for a while now. Out of the two skeleton brothers, you had gotten to know Sans the best. Both of you had become comfortable in telling each other secrets. It didn’t take long for the two of you to realise you both had a thing for weight gain and feederism. He’d notice you side-eyeing him whenever you had dinner; trying to see if the food he devoured was making any immediate effect on his magical belly (which blew your mind when you discovered it). You were a bit lighter than Sans, but still had a noticeable tummy and some wide thighs; something he lacked from being a skeleton. He was fine with putting all energy he consumed into making his blue belly rounder and squishier.
When Papyrus wasn’t home, you and Sans would experiment to see how much food you could both put away before you got full. Every time Sans would beat you without fail. You were happy to watch him pack it all away and feel as the curve of his belly got rounder with each mouthful. Once he had reached his limit, Sans would fall asleep and let you use his belly as a soft pillow. Whenever Papyrus got home and saw you lying like that, he dismissed your activities as just remnants of snacking while watching movies together.
Nothing sexual ever happened between you two, but this kink was private. Papyrus was so innocent you could enjoy doing this under his roof and he never suspected a thing. At least, he never questioned either of you about it.
One afternoon when you would usually enjoy watching Sans pig out, you walked into the lounge room and found no food lying around. Sans asked you to join him on the couch and you sat down, intrigued.
“Hey, human, I wanted to make a suggestion.” Sans started.
He looked casual; you couldn’t tell if he was about to pull a prank or pun on you.
“I’m listening,” you say.
“You know how you can never seem to eat more than me? I want to try a trick on you that I sometimes do to myself when I’m...bored.” Sans said, averting his eyes.
“O-ok, what is it?” you ask, a little wary.
“I discovered I can fill my own belly up with my magic to just bloat myself. It’s not fulfilling, just the sensation of growing big is pretty cool. I’ll demonstrate just to show it’s safe.” he said, a little more excited.
Sans sat back and aimed his hand at his belly. His hand glowed blue, like when he uses an attack, and started concentrating. After a few seconds his belly started expanding and the deliberately tight shirt he was wearing started riding up. You cupped your hand to your mouth in amazement as you watched his blue belly get darker with the input of swirling magic.
You were so transfixed by it you only snapped out of it once Sans stopped filling himself up. His belly had spilled out onto his lap and his shirt sat snugly over his chest. He ran a bony hand over the curve of his belly and let out a content sigh.
“I could keep going but I want to save my magic for you. Do you want to try it?” he grinned with a glowing blue eye.
“I, uh...wow, yes sure.” you replied, feeling how hot your face was.
“I’ve never tried this on anyone but myself before, so if you feel uncomfortable just say stop when you’re ready, ok, [name]?” Sans said sincerely.
You nodded in agreement. Sans placed a hand on your shoulder and gently pushed you back on the couch so you were in a half lying down position. He edged closer towards you, but he was struggling with his bigger belly being in the way. Sans kneeled other side of your legs but didn’t force his weight on them.
He placed a bony hand over your chubby belly and concentrated. Almost immediately you felt a warm, tingly sensation inside you just as you saw your belly start to expand. It wasn’t the same as when you chugged a coke; you were getting bigger and not even feeling all that full.
“How does it feel? Are you hurting?” Sans asked you.
“N-no I’m totally fine! Please keep going.” You say, a little short of breath.
Sans kept slowly adding magic to your belly, watching for signs of distress in your expression. You couldn’t help but grin and blush from the excitement of what was happening right before your eyes. Your shirt started to get tight and as your belly kept slowly inflating it lifted up by itself revealing your belly button. It was extremely satisfying just watching and feeling your sides grow with no repercussion of pain...yet.
You tried pulling your shirt down as your belly continued to grow but it was no use. Sans was grinning wider than usual from a combination of seeing you enjoying this just as much as he was. By now you had almost doubled in size; completely unobtainable through normal stuffing or water bloating. Your belly had almost swollen out enough to touch Sans’.
Despite it not hurting, you noticed some stretch marks appearing on your sides. Rubbing a hand over your belly you realised you weren’t soft anymore, but going tight. Skin can only stretch so far after a certain amount of time. You had a look of worry and Sans stopped immediately.
“Is it hurting? What’s wrong?” he asked concerned.
“It’s not, but I think I might burst soon if you keep going.” you said with a nervous laugh.
Sans placed his hand onto your belly and felt the tightness of it. His grin faded, “I’m sorry, I didn’t realise this could happen to humans.”
“Heh, look, it’s fine. And you see these?” you pointed out the stretch marks lacing your swollen sides, “Humans get these when we’re stretched out too much.”
“Oh, ok, I’ll watch out for those next time.”
“Nah I don’t think they’re going away now.” You shrugged.
“Just as long as you’re ok,” Sans smiled again and ran both hands over your tight belly.
You let out a content sigh. Sans spent the next few minutes rubbing his bony hands all over your new huge belly.
With half lidded eyes you ask, “Will the magic wear off soon?”
“It should vaporise soon enough. But not before Papyrus comes home I think. We better get you to my room before he sees you and worries.” Sans said, already slinging an arm around you to help you get up.
It was a struggle to stand with your new centre of gravity. With Sans’ help you slowly waddled over to the stairs. Sans’ magic was spent so he carefully guided you up the stairs. Your belly wobbled with every movement, but it didn’t make you feel sick. It was a good experience and you trusted Sans you would revert to your smaller size later.
Sans aided you onto his bed and sat by your side, gently rubbing until he heard the front door open. You smiled up at the short skeleton and thanked him for doing this with you. Before he left he leaned over and whispered in your ear.
“Next time I can show you how big I can get with magic.”