This story based on a fanfic that I saw.
Everyone on the Underground had a child. Wingdings Gaster was a skeleton man that wanted to know why the Underground didn't have more skeletons, but at the same time, he wanted to know how was taking care of a child.
Gaster walked to a lake where it was called Magilakoon, where it had a pink shining water.
"I want to know, how do I create a monster that is like my specie?" Gaster said.
A purple fennec girl appeared in the lake.
"All you had to do is use one of this transparent bubbles, put the bubble in the water of the lake, and a part of your skin or bone inside the water, and then the monster of your specie will be created." The fennec girl said.
"But inside the bubble, the baby still can eat?" Gaster asked.
"Yes. A yellow cord will be on it's hand. When the creature is ready to born, the bubble will pop it by itself. Don't worry, nothing can pop the bubble." The fennec girl said, disappearing.
Gaster did everything what the fennec girl said to him.
07/04/1992
The fetus is only a white slime. I have to wait some months.
07/26/1992
The fetus has gained some form. It's turning into a creature.
08/16/1992
It has a form now. It's a small skeleton.
Gaster was on his desk, writting on his diary, when he heard a sound of popping.
Gaster looked back only to see a small skeleton on the floor, with a cord tied on his left hand.
The baby started to cry.
Gaster cleaned the baby and cut the cord of its hand.
Gaster discovered that the baby was a boy.
The baby was crying, but his mouth didn't open, only his teeth moved.
Gaster grabbed a bottle for the baby.
The baby grabbed the bottle and started to drink it, without opening his mouth.
"I think he is absorbing the milk." Gaster thought.
Gaster was a little bit anxious of how the king would react if he telled him about that he created a child.
Sans was playing with his rattle.
Gaster entered on the castle. The queen and the king were on the couch.
"Mr and Mrs. Dreemur, I have something to tell you..." Gaster said.
Gaster telled about how he created his first son, and Asgore didn't mind about it.
Sans was sitting next to his father, holding a plush of a red cat.
"Da-da-da..." Sans said.
Gaster looked surprised at Sans.
"You're trying to speak dad?" Gaster said.
"Da-da-dad!!!" Sans said.
Gaster hugged his son, proud of him.
Gaster created another child when Sans had 3 of age.
It was Papyrus.
After years, Gaster was bringing some groceries, when he slip and fell, falling to the ground and dying.
When Sans tried to wake his father and noticed he died, he started to bawl.
"NO!!! IT CAN'T BE!!! HE DIED!!!" Sans shouted.
Sans had to live with Grillby until he had 19 and decided to work.