1.

Sans didn't often do laundry, and he was starting to realize that that might be a not good thing. He was a skeleton, so body odor wasn't a problem, but he did have a lot of old ketchup stains on his clothes, and he should probably do something about that now that he had a real job that wasn't sitting in a cardboard box waiting for someone to walk past all day. The lab did have a laundry room in an otherwise fairly deserted corner of the lower level.

The only problem was their size. They were top-loaders, and just about Sans's height. He managed to cram most of his ketchup-stained clothes in by standing on his tiptoes, but then he still had to turn the thing on. He got a running start and managed to jump on top of it. There. He twisted the dial and it began rumbling in a comforting sort of way. He didn't really feel like getting down.

"Sans?"

He blinked, realizing that he must have fallen asleep. Gaster was shaking him.
"Are you alright?"
"Huh? Oh yeah. Fine. 's nice and warm up here."
"How did you get…" Gaster stopped himself and instead lifted Sans to the floor. "Never mind." He paused. "Your load is done. Can I use the washer? All the rest are full."
"Yeah sure. Uh, would you mind moving out my stuff? Otherwise I have to jump up there again."

2.

Gaster didn't mind, but now that he'd thought of it, Sans realized that unloading his clothes from the washer could be a problem. And sure enough, when he did his next load (because now that he actually looked at his clothes he realized that all of them needed to be washed) it was. There wasn't anyone nearby and he was too proud to go upstairs and ask someone who was probably busy doing important science things to come help him because he was short. He could do this. He pushed the lid up and hauled himself up over the edge of the machine, so that his head and arms hung down into the warm, damp interior. There. That worked just fine.

….Up to a point.

There was one sock plastered to the lowest bend of the furthest away corner. Of course there was. He wriggled further in, swearing under his breath, and snatched it. At the same moment he lost his balance and slid forwards. He managed to stop himself from falling into the washing machine by jamming his head against the agitator, then wriggling out backwards, still holding the sock. Once safely on the solid ground again he sighed. That had been more trouble than it was worth.

It occurred to him that he could have fit very comfortably into that washing machine.

It occurred to him that he had just been given the perfect material for a prank.

…Or a nap, if he really didn't want to be found. But so far he wasn't that desperate to avoid his coworkers.

3.

Sans spent the next few days listening for someone to say the word 'laundry.' When it came, it was like a personal gift from God, dropped into his lap. He was talking to Yoro, the wolfish night watchman, who suddenly stopped mid-sentence to say
"Ah man that's another thing I need to do tonight, I have to do laundry. I'm out of pants."
Sans didn't wait. As soon as the conversation was over he went to the laundry room, opened the lid of the first washing machine, and slid in over the edge. After a bit of tumbling around, banging knees and elbows, he got himself into a sitting position with a leg on either side of the agitator, his back against the side of the interior. It wasn't bad. He could fall asleep here. He wasn't going to, of course. He was going to be ready when someone came in. He was going to be ready….

It was very dark and warm with the lid mostly shut. He'd jammed a sock under it so that it let a little air and light in, but still…

He woke up to the muffled sound of voices. He'd fallen asleep with his chin in one of the ridges on the agitator. Quickly he straightened up, knocking the lid back with his head.
"Heya!" he shouted, realizing at the same time that these weren't the prank victims he'd been expecting. It was Gaster and Grillbz, having a conversation about something terribly intellectual—at least, Gaster was talking and signing and Grillbz was watching him. Grillbz had been chilling at the lab for the past several days. Sans wasn't entirely sure why, but Grillbz gave the impression that it was to stop Gaster from sticking his fingers in electrical sockets and calling it science. At Sans's appearance, Gaster screamed, tripped over a laundry basket, and disappeared from view with a thud. Grillbz blazed white hot and Sans instinctively ducked.
"Oh. Hey. What are you two doing here?"
".was that supposed to be funny?" said Grillbz, slowly cooling back down. Sans grinned uncomfortably.
"…yeah."
".scaring people is funny to you?"
"To be fair I wasn't expecting you two, I was expecting someone else."
For a monster without visible features, Grillbz's glares could be very expressive. Sans slowly sank back down into the washing machine.
"I'm sorryyyyyy." His apology was spoiled by the fact that he was giggling nervously. Outside, he heard Gaster get up with a gasp.
"I've got it!"
".what?"
"I just remembered a thing!"
Feet rushed out of the room.
"I must write this down!"
".Gaster no."
"Gaster yes!" said a faint, retreating voice. Grillbz sighed hot air.
".why are you like this?"
Sans made himself stop giggling and peeked out of the washing machine.
"Uh. Hey. Could you, uh, lift me out? I'm a bit stuck here."
Grillbz looked at him for several seconds.
"…Please?"
".no."
"What?"
".you're an asshole. stay in there."
"What no! You don't understand I'm short I can't get out!"
".have fun."
Grillbz walked out and the door swung shut behind him.

Sans realized he'd made a terrible mistake.

4.

Yoro would be able to get him out, whenever he came in. It had to be sometime that night. Right? Unless he procrastinated, like Sans himself was constantly doing.

Hours passed. He was starting to feel very cramped, which was a real feat for a monster without physical musculature.. He'd spent some time standing up in the washing machine, shouting, in the hope that someone would be walking by in the hallway outside. He'd tried climbing out himself, but he was in too awkward a position. So he was back to snoozing inside the washing machine, chin against the agitator.

He didn't think Gaster would have left him in there, even though he had scared him, but Gaster had a tendency to forget things when he was working, so he couldn't expect help from that area, given the eureka moment he seemed to have had while lying on the floor.

Finally Sans was roused from an uncomfortable snooze by a faint sound from outside the washing machine. He raised his head listening.
"nev…on….ive….or…youuu…."
What?
"nev..gon….ound and…..sssert youuuu…"
Oh. It was someone singing, quietly, almost in a stage whisper. Sans stood up, pushing the lid back.
"Hello…."
It was the kodama, whose name he could never remember. The one who always seemed vaguely uncomfortable when you looked at him, but was otherwise pretty normal. He had headphones on and his back to Sans, and he was dancing in a slow, contrived, and rather bizarre way, waving his legs and arms and shuffling around.
"never gonna make you cry, never gonna say goodbye…"
"Hello?" said Sans, a bit louder.
"never gonna tell a lie and hhhh…."
The kodama turned and caught sight of Sans, who waved.

The kodama looked like he had been caught wearing underpants on his head. He inched backwards towards the door, eyes very large, mouth tight.
"No no wait!"
He sidled out into the hallway and Sans heard the sound of small feet racing away before the door closed behind him.
"But you just said you'd never desert me, you liar!" Sans shouted, then collapsed back into the washing machine, sobbing with desperate laughter. He felt like it was going to be a long night.

5.

He was fast asleep when Yoro came in at around two in the morning, and only woke at the sound of something heavy being thrown down on the washing machine next to him. Before he could bring himself to stand, the lid of the washing machine he was trapped in opened.
"Oh god the light! It burns!" said Sans. There was no response. He looked up. There was a sound of tuneless whistling, then a pair of socks flew through the air and landed on his face.
"Hey!"
His voice was muffled by the washing machine. A pair of shorts followed the socks. The whistling continued. Is this how I die, he wondered? He straightened up with an effort, and there was Yoro, digging through a laundry basket on the washing machine next to him. Sensing motion, he looked up, saw Sans, screamed, and reflexively flung the laundry basket at him. Sans also screamed, flailed, and somehow managed to catch the laundry basket with his still-developing telekinesis and fling it to the side.
"SAAAAANS OH MY GOD!"
"Please get me out of here."
"You nearly gave me a heart attack!"
"I'm sorry. I've been in here for hours."
"What? Just to prank me? Dang, that's some devotion."
"No you don't understand I'm stuck. Please help." Sans hopped up and down to demonstrate his stuckness. Yoro stared, then doubled over in laughter.
"Or, I could just leave you."
"No! I only have one HP, I need to eat!"
"Haha—wait, what?"
Yoro froze. Sans sighed. He tended to forget how horrified other monsters were when he mentioned his HP problems.
"Just please take me out of the washing machine."
"Geez man, I knew it was low but I didn't know it was that low. Are you OK?"
"I will be when I'm not stuck in a washing machine anymore!"
"OK, OK." Yoro lifted him out and set him gently on the floor. "Hey, that was good. I'll need to get you back for this."
Of course.

6.

Sans was able to wash his last load of ketchup-contaminated clothes without incident. He didn't get stuck in the washing machine, he didn't have anyone laugh at him, everything was great. Until he came to unload the dryer. He was folding a shirt, humming quietly, when there was a sudden sort of intense silence, the lights flared, and then everything went black.

"Sans?"

What. Where…?

"Sans! Sans no!"

There was a bright light shining in his face. He had a pounding headache.

"Saaaaans!"
"Ugh. Hey. So do I follow the light, or…"
"Oh, you're alright. Thank goodness."
Sans sat up. He was in the darkened laundry room with unfolded clothes scattered around him. Gaster was shining a flashlight in his face.
"What happened?"
"Er…" Gaster looked uncomfortable. Behind him, Grillbz walked into the room, which became much brighter with his presence. Gaster clicked the flashlight off.
".good question. what did happen?" asked Grillbz. Gaster cleared his throat—an unnecessary action for a skeleton.
"I ah…"
". Gaster. . what did you do."
"The Core prototype looked like it was working so I hotwired it to the lab to see what would happen. And um. The good news is that it works."
".what."
"It works so well it blew out all the circuits in the building, as far as I can tell."
Grillbz stared at Gaster, who pulled his turtleneck over his face and began laughing weakly.
"I am so sorry. I should be locked up for this."
Grillbz silently picked up Gaster, walked to the washing machine Sans had previously been stuck in, and crammed him into it. Gaster was still laughing. Grillbz shut the lid. It wouldn't shut all the way because Gaster's head was in the way.
"I'm sorryyyyyyy."
".stay there and think about what you've done.I'll take Sans to the med wing."
Grillbz tucked Sans under his arm and walked out.
"At least give me my flashlight?" said Gaster breathlessly from the washing machine. "Please?"
Sans telekinetically tossed it to him as the door was shutting behind them. It hit him in the face. Gaster caught it, and started laughing again.

A/N: Have you ever had a day where you just need to write something stupid to make yourself feel better? Voila. I hope this makes you as happy as it made me.

…I think I just find washing machines inherently funny. I don't know why.

The rick-rolling kodama is a relative of the ficus-licker and eventually probably becomes one of the Goners.

This ties in to COREverse, loosely. You can view it as canon or noncanon to the main story depending on how you feel about that. Probably noncanon because yes, Uncial Gaster does some stupid things, but I don't think he would hotwire an otherwise untested prototype to the lab with all of his assistants inside. He'd not a psychopath. He gets overwhelmed with feels and starts sobbing when you hand him a baby, that's literally the opposite of a psychopath.