Sans is a MONSTER and not a HUMAN, so he doesn't follow HUMAN biology like a HUMAN skeleton. Same with Gaster and Papyrus. 

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"I really can't believe how much they've grown." Gaster sighed, sitting on the couch in Frisk and Sans' house. Heck, even Grillby was here too as once in a blue moon everything was closed down for the day. Literally no one was working today at the Mafia headquarters because Asgore had shut everything down for just today. Probably because he was trying harder to spend time with his family. Well, actually, it used to be once in a blue moon he would do this, but it seemed to happen more often these days. 

"tell me about it. half the time i can hardly believe it." Sans murmured, going to sit down on the other couch next to his girlfriend, staring towards his and Frisk's two children who were currently playing in the middle of the room. He had decided to pick up the small table and move it out of the way earlier this morning, knowing perhaps it was better to do so, so that they had more space to play down here. Gaster smiled. 

"What's new with you two anyway? Things have been so busy with these two and just work that... We hardly get the time to sit down and talk as often." Frisk mentioned. Because when Gaster was around to babysit, they might have come home from being tired from work, and neither of them would really be in much of a social mood. It was nice that they had one of those opportunities where they were able to catch up. Sans also knew that he wanted to hear about how things were going for his father and Grillby. They were still together going strong after these few years after all. 

"That's very true." Gaster nodded, before he smiled towards Grillby. Sans seemed to raise an eyebrow at a knowing glance the two seemed to share, and it was then that the flame monster appeared to show his left hand, and the skeleton and human could hardly even hold back their gasps - Frisk's was probably the loudest out of the two of them as she practically leaned forward in her seat to look. Grillby was wearing... An engagement ring! No way! 

"When?!" Frisk gasped, being so overjoyed for the two. They always seemed so happy together, it was about time that they decided to make it official. Sans was just sitting both in shock, and in amazement. He just knew that he was happy for his dad that he seemed to have the courage to get remarried. Sans truthfully never thought that his dad would want to go through being married again, seeing as he already had a wedding before when being with his mother. But Grillby must have been so important to him that he decided: why not? Sans wished he could gain the guts to just buy a ring, get down on one knee, and ask the girl he loved to marry him, without overthinking it so damn much. 

"Just last night. He took me on quite a nice date. I just didn't expect him to take me by surprise and cause me to cry on said date." Grillby joked towards the oldest skeleton in the room, who let out a small chuckle of his own. The flame monster truthfully just thought the skeleton was trying to be cheesily romantic, little did he know that Gaster had other plans, and he had actually got down on one knee right in front of him. He never would have expected himself to act in the most cliche way possible, and that was throwing his hands over his own mouth as he started to cry tears of joy. 

"Awh... That's so sweet..." Frisk smiled. "Which one of you do you think is going to wear the dress?" She teased with a giggle, causing the two other monsters to glance at each other. Grillby rose his eyebrows at Gaster, while Gaster just seemed to raise one eyebrow at him in return. That was the moment the scientist decided to glance away and he cleared his throat, gradually beginning to blush. If anyone could successfully manage to break that poker face of Gaster's, it was definitely Grillby. 

"i think they're both fine with suits." Sans chuckled, leaning back into the couch. "but really, congrats ya two." He really was happy for them. Though, part of him wasn't as excitable, only because he knew something like this was going to give people another reason to pester him about the fact that him and Frisk weren't yet married. Again, it wasn't that he hated marriage, it was just... Ugh, what was the point? Point was, he was too afraid, and it made him feel stupid. He felt stupid enough that he had admitted his fear to Mettaton, and that Chara seemed to pick up on his terror. 

"Well, I guess so..." Frisk said. She actually wanted to ask Gaster where he had ended up getting his ring from for Grillby; due to this conversation, she had started to think about what Chara had said to her, about how maybe she should just be the one to ask Sans. She knew she had really wanted him to ask, but if he was too fearful, maybe she would just have to be the one to do it. Seeing his reaction would make her just as happy if he had asked her. "Sans, can you grab me a snack from the kitchen? I'm a little hungry, and I want to continue talking to them about their engagement!" 

"huh? ya could just get up and come back..." Sans murmured, but seeing the pleading look on his girlfriend's face, he exhaled. Sometimes he hated that puppy eyed look. When it came from Frisk, she probably could convince him to do anything. "fine..." Sans picked himself up off the couch, and went towards the kitchen to grab a snack that Frisk asked for. "what do ya want though?" 

"Anything would be fine-- Oh, actually, no, can I have a sandwich?" Frisk politely wondered, as she wanted to keep him preoccupied at least for a couple of minutes, and a sandwich was just the first thing that came to mind. Sans just nodded and continued on his way to make his girlfriend some food, and that's when she lowered her voice towards the other skeleton monster and the man made of flame. "So, um... Where exactly did you... Get your ring? If you don't mind me asking..." Frisk didn't want to seem rude for asking if he didn't want to say. Gaster grinned. 

"I already told Grillby this, so it's fine. But there is an exceptional jewelers in town that I decided to get the ring from. Why? Are you thinking of asking someone a question?" The skeleton teased, seeing Frisk blushing a little in embarrassment. This was Sans' dad she was saying this to, and it wasn't until she took the time to think about it that she decided perhaps it might have been less embarrassing to ask someone else. The skeleton chuckled. "If you want to ask Sans, you of course have my full blessing." Gaster told her. Why wouldn't he accept such a thing when he knew that Frisk made Sans happy, and they already had a family together? It was only right for them to want to continue their future together forever. 

 "W-Well, thank you..." Frisk weakly smiled. "Could you maybe one day show me where that jewelers store is?" She asked quietly, seeing the other skeleton nod, not seeming to mind at all. After their small conversation, she definitely found it difficult to remove the beaming grin from her lips, even when Sans had returned. He would probably just assume she was smiling out of happiness for Grillby and Gaster though, (which was part of the reason why the edges of her lips were still being tugged at). "Thank you, Sans." She said, when she was handed the plate with a sandwich on it.

"no worries." Sans grinned back at her, as he plopped back onto the couch. It was then that him and the other two broke into conversation, while Frisk began eating her food. It wasn't until she actually really took the time to think about it that she felt her heart do a double take. Wait, did this mean she was really going through with it then? Was she really going to decide to do this? 

Was she going to ask Sans to marry her?