Doctor Wingdings Gaster walked into his lab. He used his hands to sign to himself as he said, "I'm going to make some children to break the barrier! This is flawless logic and nothing can go wrong!"
He walked over to one of his machines, and cut a hole in his right hand. "There, I am going to clone a kid from the genetic material of my right hand, instead of something less painful or at least less prominent. It doesn't even have stem cells in it! Wait, do skeletons even have stem cells?" He shrugged. "Whatever, I have a hole in my hand now. This is fine."
The skeletal scientist dropped the cut out piece of his hand into a giant cloning tube he made specifically for this purpose. "Project 'Let's make kids to break the barrier' Prototype One has started. ...You know what would be a good idea? Injecting the kiddo with Determination I extracted from a human soul!" He grabbed a bubbling vial labelled "DT" and attached it to the cloning tube, looking at the readings as his hand piece began absorbing the red liquid.
"Welp. Now to wait for the cloning process to start." So he sat there and watched as the machine somehow allowed his hand piece to begin forming a skeleton child, as if it were a mechanical womb. It took a long time, and Gaster may or may not have forgotten to eat or sleep throughout the whole process.
The readings of the skeleton child were unusually low, and the child looked pretty much nothing like him, with a round face, a toothy grin, and wide eyes.
"Oh no! The vital signs are really low! This might be because I shoved some Determination into him before it was born!" He was already starting to get confused on whether he should call the experiment a person or a thing.
He worked really hard to siphon out the Determination as soon as possible in a very dramatic scene of panicking about losing his valuable experiment. But that didn't really change much, and the kid still had only 1HP. Oops. At least he wasn't melting!
He left to get some popato chisps, as the kid was still developing and wouldn't be ready to take out of the tube thing until later. So he walked to a random vending machine in Hotland and bought himself some chisps, eating them.
When he returned... THE CLONING TANK WAS EMPTY!
Dr. Gaster dropped his chisps and screamed. "OH NO! HOW MANY MISTAKES CAN I MAKE CLONING ONE SKELETON KID?!"
At that moment, a skeleton child had walked over to his feet where the bag fell and was eating the chisps, shoving them into his face as if he'd never had food before. Which was true, because he spent all his life in a cloning tube.
Wingdings stopped screaming and looked down at the kid and said, "...How the fuck did you get out of the tube?" He picked it up and said, "Okay, you're gonna go back in there and grow up or you're gonna have a bad time and be short for the rest of your life."
The naked skelechild squirmed, and his left eye flashed blue and yellow as he turned Gaster's soul blue, pulling it to the ground.
"You wanna go, kid? Huh? You wanna go? Okay," he complied, as his own left eye started flashing, blue, green, purple and orange, "Let's fucking go."
He grabbed the child with his telekinetic magic, picked it up, and dumped him back into the cloning tube. The doctor held both of his floating hands up, middle fingers sticking out. "GEEETTTTT DUNKED ON! How'd ya like them apples?!"
The kid disappeared and then reappeared outside of the tube.
Gaster's face fell and he looked at the kid.
He picked his head off the floor and screwed it back onto his neck. "Oh dear. I did not think this through. This is bad."
As he was saying this, the little skeleton began climbing Gaster.
"Oh shit I have a kid now and I am getting attached to him." Gaster had finally decided to continue calling skelekid a him.
"You need a name, kiddo." He picked the kid off of him and set him back down, then looked at the cloning tank, which was now devoid of skeleton.
"Well, the tube is now sans-child so Imma call you Sans, alright kiddo?" Gaster rubbed the top of the kid's head.
The skelechild made a valiant attempt to speak. "S-ss-sss-ss!" Gaster was pretty sure the kiddo was trying to pronounce his name.
Gaster smiled. "Nice try, son. I do not regret making you and pretty much fucking up half of your creation accidentally. You're adorable."
One day, Asgore found out about the little skelekid and said "Okay, Dr. Gaster, I know you're the royal scientist and all but I don't think you're responsible enough to raise a child. Children shouldn't be experiments. That's bad, and if you keep trying to make the kid break the barrier, you're bad too."
Gaster responded with "Nah he's my kid and you're not taking him away from me, besides, I know you hate killing the humans who come down here, and if I succeed, you can stop killing humans!"
Asgore didn't want to be mean, so he conceded. "Okay. Don't be mean to your kid, alright, Wingdings?"
Gaster nodded and went back to doing his usual science.
Time passed, and Gaster raised Sans as his son. When people asked where he came from, he said "I made the kiddo in a lab, lol." Everyone laughed, and just assumed that he wasn't on good terms with the child's mother. They also asked were the hole in his right hand came from, to which he replied, "I cut a hole in my hand so I could make Sans here. He's my right-hand man!" This was also taken as a joke, and everyone just assumed that this came from some sort of embarrassing accident or something in his crazy lab for crazy people.
"Hey kid," he asked one day, "Want a brother?"
Sans just looked at him for a moment, then said, "what."
"A brother," Gaster explained, "Another skeleton, sort of like you, but not short and fragile. I made you to break the barrier, remember? But you only have one ATK, one HP, and one DEF. So I wanna make another kid. So you can have a playmate. And he might be able to break the barrier and save everyone or something."
Sans rolled the little white pinpricks of light that served as his pupils and said, "well you're the scientist, dadster. and please don't fuck up my bro. y'know what? i'm gonna go and supervise your nonsense. besides, don't you need the power of seven human souls to break the barrier?"
"Yeah," said Gaster, "That's why I tried to give you Determination. So you'd have that power."
"yeah no, that almost made me melt, and if you had seven human souls worth of determination, we'd already have seven human souls to get it from and making kids to be your barrier breaking experiments is stupid and pointless. besides, you said that you're not going to make my brother as fragile as me so you're not gonna put DT in him. so why do you wanna make another skeleton kid?" Sans crossed his arms and looked at his Dadster. He was still short, even though enough time had passed for him to learn a whole lot of science shit as well as everything he needed to know about himself.
"Well um..." Gaster summoned two more floating hands to wring each other, because his nervous tick was summoning random floating copies of his hands to communicate while he spoke, "...you look sort of lonely when I'm busy science-ing. So I want to give you a brother! AlsoImessedupwithyouandIthinkIcandobetter."
Sans raised a nonexistent eyebrow and said, "k."
So Gaster went and cut a hole into his other hand and plopped it into the cloning tube. He didn't put any DT in, and made sure this second kid didn't have his teleporting powers. So it took a while, and the kid grew taller than Sans.
Papyrus was a sweetheart from the moment he was taken out of the tube. He constantly tried to hug both Sans and Gaster, leaving Gaster to ask "Was I ever this clingy as a kid?"
Papyrus was also really fucking loud. He didn't know how to talk quietly and always yelled absolutely everything.
Gaster just named him Papyrus for no reason. Because why not.
Their typical conversation would go something like: "HEY DAD! CAN I GO OUTSIDE AND PET DOGS?" to which he would respond from his lab, "KEEP YOUR VOICE DOWN, YOU DON'T NEED TO BE SO LOUD ALL THE TIME!" This would earn him a "WELL I CAN'T HELP IT! I CAN ONLY SPEAK IN ALL CAPS FOR SOME REASON!" Sans seemed to like this and would always talk about how his brother was "so cool."
One fateful day, Gaster was checking up on some science on the core and he tripped and fell into it. He tried to teleport out, but somehow got sucked up by the Void. He then became scattered across time and space, leaving Sans and Papyrus orphans. Nobody remembered him. The end.