Long ago, way before the events of Undertale, Sans was heading home after an amazing field trip to the castle in New Home. All that went through his head was how he got to meet the king himself. Asgore was incredibly nice and instantly recognized Sans. "You must be Gaster's son. I've heard so much about you." He said to Sans and all of his classmates stared in awe, amazed to know that the king actually knew him. It was definitely one of the best days of his life.
He reached home and immediately went to his father, "Dad, Dad, Dad! Today was the best day of school EVER!" The older skeleton giggled at the hyper-activeness of his son. "Quiet down, Sans. Your brother is sleeping."
"Oh, sorry"
"It's okay, Sans"
The young skeleton calmed down and told Gaster about his day at school. After Sans finished, he asked if he could visit again. "Well, maybe next time I go to see Asgore, I might bring you along." At the sound of that, Sans began to jump around excitedly but stopped when he remembered that Papyrus was still sleeping.
But it was too late when both skeletons heard a distant noise from the other room. Gaster went immediately to get Papyrus. When he brought the small baby bones in to the living room, a smile covered most of Sans' face.
"Hey little bro"
"Hai Snans!"
Sans smiled at the sound of the voice of his little brother. It was adorable. He hoped Pap would always stay this small. He was probably hungry. Sans was also. Gaster cooked some spaghetti for the three of them.
After dinner, Papyrus was sent to bed. Sans was also getting ready to sleep when Gaster stopped his son. "Wait. I want to show you something." Gaster took Sans to his lab and he was in awe by all of the crazy gadgets and trinkets laying around. Sans was so amazed that he didn't even realize he walked straight into a wall. "What is all of this" Sans asked.
"Well, if I were to talk about every invention here, we would be here for 5 years"
"Okay. Then what about this one?"
Sans pointed to a separate room with a window. Gaster explained what it was.
"That is one of my newest projects. It is to study the science of timelines."
"What are timelines"
Gaster explained the basics of timelines to Sans and he seemed to understand it.
"Even with everything I told you, this is relatively new research. All this information is just an educated guess.''
Sans was very interested in this and he really wants to help.
"Can I help with this?"
"Well… I guess you can watch b- "
With that Sans was excited and grabbed a lab coat. Gaster was happy to see him so enthusiastic but hoped Sans would get bored. His work is very dangerous and Gaster would never forgive himself if Sans got hurt. Afterall, Sans is still very young, only seven. He knew it would only be a phase and Sans would grow out of it.
Sans was able to find a lab coat that was just his size and quickly put it on.
"So, what will we do first Dr. Dad" Sans said. Gaster grinned at the sound of his nickname.
" I think you should go to bed first. You have school tomorrow" Sans reluctantly went to his room and fell asleep.
Gaster spent the rest of the night doing more research. He noticed that the timelines can be overridden by a power he discovered, called 'determination'. Only humans had this power, but what if there was a way for monsters to harvest that power and possibly 'reset' to before they were trapped underground. Gaster suddenly came up with an idea. He wrote some blueprints and then left to go to bed.
The next day, when Sans got home, he immediately did his homework then went to the lab. He noticed that Gaster was building something new.
"What are you building?" Sans asked
"This is the 'Determination Extractor', 'DT Extractor' for short. It is made to take the determination from this"
Gaster held a red heart. Sans knew it was a human soul.
"How did you get that?"
"I told the king that by extracting the determination, I could put it into monsters and I would be able to harvest their souls when they turn to dust." Sans remembered from school that a monster soul dissipates when they die.
"Can I help build it?" Sans asked
"I guess. It's not dangerous right now"
Sans smiled and grabbed some of the materials. The skeletons worked until they finally finished building the exterior.
"Sans, this is very dangerous so I would advise that you wouldn't help here. Maybe you can play with Papyrus." Gaster told Sans. Sans agreed and went to his and papyrus' room.
"Sup bro" Sans told his two-year old brother.
"Hai!"
"What you doing?"
"Drawling"
"Really? Can I draw something too?"
"Yah!"
Sans and Papyrus drew pictures. Papyrus drew scribbles on the paper while Sans drew himself, Pap, and Gaster. Happy with what he drew, Sans put it into the pocket of his light blue jacket. When Papyrus finished his drawing, Sans took it to the kitchen and put it on the refrigerator.
Sans thought Papyrus must have been hungry, so he cooked some tacos, the only thing he knew how to cook, and ate dinner. He made extras incase Gaster would finish his invention.
After Papyrus went to bed, Sans went to the lab to check on his dad. When he entered, he saw that Gaster was asleep. Sans put a plate of tacos beside him and left for his room to sleep.