soiburger:

Summary: Sans is hooked up to a water tube and strapped into a chair in Gaster’s lab. It turns out Gaster had other intentions instead of science and Sans ends up consuming more water than he had intended.

***

Sans did everything as he was told. He sat obediently in the chair in the middle of the room and let the doctor secure him in with tight leather straps over his ankles and wrists that bound him to his seat.
“What’re these for?” Sans tested his freedom whilst Gaster’s back is turned.
“My machines are very delicate. Those restraints will ensure you won’t damage them.” The doctor removed his lab coat and smoothed his black sweater into pristine condition. He turned, catching Sans messing with the straps and rolled his eyes. “Please stay still whilst I install the tube.”
Sans narrowed his eyes,
“You mean I have to have that pipe…in me?”
“Yes. It shouldn’t hurt.” Gaster gently blew on the end of the long black pipe and smiled. “Now, open wide.”
Sans did so, opening his jaws to the full extent and watching his mentor feed the tube through it.
He winced, the plastic scratched the back of his throat and began to slide down his oesophagus. It was a gross sensation, as if a snake was slowly slithering down instead of a pipe.
“Gu sure gis is save?” Sans gargled, setting off his gag reflex. “Calm down!” Gaster tutted, “Of course it’s safe.” With that, he shoved the tube the little extra way in it needed. Sans grunted, the tube was now inside his stomach. When Gaster turned to program the machine, Sans gnawed on the obstructive object in his mouth, finding it both uncomfortable and entertaining.

“I trust that you are ready to begin?” The tall monster looked under his glasses at his subject, a little smirk finding is way onto his lips.
“Geah.” Sans mumbled.
“Wonderful. Then I’ll turn on the machines.”
Gaster flicked a switch and the room filled with a consistent low humming sound.
Sans could also hear the trickle of water as it rushed towards him via his attachment. It was somewhat ominous as the running water became louder and louder, closer and closer. Sans stared- wide eyed- at the structure protruding from his mouth.

“Well, if you need me, Sans, I’ll be in my office.” Gaster yawned and tucked a stack of folders under one arm.

“Gore Geevin’??!!!” Sans spluttered, he really didn’t want to be alone whilst this happened. What was this experiment meant to prove anyway? He decided not to think on it too much.

“Yes. Why? It won’t be painful in the slightest, my boy. Just sit tight and wait for me to come back.” And with that, the doctor slotted his glasses into his shirt pocket, leaving the room and shutting the door behind him. Sans was entirely alone. He sighed. The water hadn’t come through yet, and so he waited for it like a train about to run him down.

He gasped, a sudden icy chill slithering up his spine.
Freezing, shocking liquid splashed inside his stomach, sending little chatters to his teeth and shivers everywhere else. From then on, the water crept ever so slowly upwards. Each millimeter it gained he felt. His tummy had been so warm before…yet as time went on he became colder and colder. He balled his fists and brought his thighs together in stress. Where had Gaster got this stuff?! It was like ice!

Sans moaned and squirmed, disliking having no control over his slowly filling belly. It felt so unnatural and forceful he could only think regretful thoughts.

“HaAhhhh…Aahhhhh!”
The liquid had touched the very edge of his lower ribs; the highest it could’ve gone. He could only round out from then on. His stomach made a little upset noise, confused and bothered by the fact that it was full of water. “Shhh…shhhh, buddy, its alright.” Sans winced, physically feeling his waistline grow. His belly bubbled in response, clearly disagreeing with him.

A few highly uncomfortable moments passed before his body began to naturally heat the water inside him. It was at this point that he relaxed, sitting back and simply feeling his body swell. He was just about at the ‘full’ stage. His ‘skin’ had a satisfying tightness to it and his belly was just heavy enough to feel good. At least now he was warming up again, a steady slosh told him that someone else was feeling a little happier too.

“Mmmmmmhhhhh…” He sighed, reclining in his chair heftily. I could get used to this. Any minute now, Gaster was going to waltz in, switch off the machine and remove that infernal pipe…

But he didn’t then. Nor did he for over another five minutes. Sans opened one eye, staring down at his heavily distended middle.
His sweater was looking pretty snug, no creases there. He noticed that his pants were becoming a bit restrictive as well, the waistband had started to cut into his soft, malleable flesh. He felt like a water balloon…though one that was in danger of bursting. His stomach continued to inflate. He watched in part awe, part horror as his sweater grew tighter and tighter over his growing stomach. His tongue materialized without permission, releasing waves of gooey drool that quickly flooded his mouth. Oh man…not now!

Sans sealed his ‘lips’ over the tube to stop himself making a mess and grunted as he tried shifting his weight to get comfortable.
The stream of water spluttered and reduced to a babbling trickle… He thought it was over. Maybe the reservoir was empty?

“GhaaAaaahhhh!” He cried, before struggling to comfort his expanding middle. The water returned to full-stream in one lukewarm gush. “Fuuhhckkkk…” his glugging stomach bounced as he squirmed, trying to remain calm. His breathing quickened, he heard tiny pops and squeaks from below and a rising heat in his pants. “Mmfff…guh…” He squeezed his thighs together to suppress his erection. Why did that have to be a product of this?!
Cool open air stroked against his newly exposed ‘skin’. His sweater had finally given up containing him and was gradually rising. He could see himself filling up, blue bubbly tummy tightening and gurgling and growing…

Yawning, Gaster opened the door and entered. He acted as if nothing was happening, rolling his sleeves up and replacing his glasses.
He looked under them at his helpless subject, raising his brows at the big blue bubble that now splurged and sloshed in their lap. Sans groaned,
“M…mgahhh…gahtdurr…ah-ahhh!” He bit down on the tube in pain, stomach reaching its limit. Gaster noted how Sans had tried calling his name and casually flicked a switch on the side of the machine.

“To be honest, Sans, I’m a little disappointed.” He mused.

The skeleton couldn’t respond, he just panted, looking upon his bloated belly with tired eyes.
Gaster knelt down and slowly pulled the pipe back up through Sans’ throat with several questionable squelches before the end popped out of his mouth with a satisfying slurp.
“There.” He muttered, “You’ve finished… Just a moment.”
The straps fell from his wrists and ankles, providing him with relief. That feeling didn’t last long, however.

Sans’ eyes widened and his pupils shrank. Gaster’s hot hands lay pressed to his exposed flesh.
“Does that feel any better?”
His fingers gently pressed on the jelly-like ball, evoking a loud groan.
Sans felt air rising quickly inside him. He tried to stop himself…too late.
“GGUUURRRHHHHPPPP”
Sans clamped his hands to his mouth, stunned.
“I…I’m sorry, man…I couldn’t…”

Gaster chuckled, standing up as if satified with himself.
“It’s quite alright, Sans.” He said calmly, “It really was a lot of fun watching this experiment.”

The skeleton rubbed his aching jaw,
“What? You weren’t here!”
Gaster laughed again,

“Was I not?”
The monster smirked and let his glasses shimmer in the harsh light. Sans was baffled.
“You’re free to go. I have no use for you now. I have everything I need to review on tape.”

***

Ok, so I didn’t know how to end that one. Hence the weirdness…If you didn’t know about the possibility that Gaster watched it through the cameras then it won’t make much sense. XD It’s so far fetched, I’m sorry!
But please request more Undertum thingies! I love writing them!
Also, if you want more vringe check out my Wattpad “Sansumeru” or “Soïburger”. I post more stuff there ^^