Sans sat there examining Frisk's memories as he had been doing since the kid had shared his memories with them.
From the last episode, Sans had discovered some more important details about his new little brother.
First, Frisk was smarter than he appeared. Child protective services on the surface were evidently stricter than in the Underground. That meant Frisk had to have known how to avoid them for almost three years, or at least, been a very convincing liar. That was quite remarkable for a child.
Second, Frisk had lied about not having anyone on the surface. He had that girl, Varda. Sans didn't know if he had forgotten or if he was only too shy to mention her. It was probably the latter. Boys tended to be shy about romantic feelings at his age.
Sans snickered to himself. He would have to grill Frisk about it soon.
Third, Frisk was slow to trust people. He was quiet around people even if they liked him. Sans didn't have enough of his memories to know if that had always been the case, or if Frisk adopted that attribute to survive, but it did explain a lot about his mental health state.
Fourth and finally, Frisk only got sassy when he was annoyed, angry, or upset. It was like a defense mechanism, kinda like Sans did sometimes…
Sans suddenly had a horrible realization and froze in fear. That was not lost on Undyne who was putting the finishing touches on the dinner.
"Sans, what's wrong?" Undyne asked. "Did you figure out something bad about Frisk?"
"Yes," Sans said in a foreboding tone. "I have realized something awful…"
"What?"
"Frisk is smart, too independent, too good at lying, shy, introverted, untrusting, sassy, tough, and strong-willed…"
"Yeah, so?"
Sans turned to Undyne and said fearfully, "Undyne, don't you get it. He's just like me."
Sans laid his head down on top of his arms while Undyne looked at him blankly for a moment.
"Oh, dear God," Undyne said. "You're right. What are we going to do?"
"I don't know," Sans said. "But one thing's for sure."
"What?"
"I know now that God has a sense of humor."