In all honesty, Sans isn’t that good at predicting people’s reactions.

He knows Papyrus will smile at his terrible jokes and then complain about smiling, but he knows Papyrus and has bounced more bad jokes off him than he can count. That’s just experience. He knows most of the monsters in the underground by now, and he has something of a handle on their personalities.

Frisk he doesn’t know at all.

So when he says-

“You’d be dead where you stand.”

-he isn’t actually intending to do more than spook them a little. He’s just being honest… though in his usual ‘just kidding’ dishonest way.

He doesn’t get to the ‘just kidding’ part before huge tears well up in Frisk’s eyes and he thinks: um, maybe that was a bad idea.

“H-hey, uh… I didn’t mean it?”

Frisk starts to cry, and Sans has now learned one more thing about Frisk: they’re a loud crier. Their sobs attract the attention of the other restaurant patrons, and they glare at him, passing judgment. He’s pretty sure he failed this one.

Sans pats Frisk’s shoulder, but that just makes them flinch away and cry even more. Out of options, he turns to his last resort: he dials up Papyrus.

Papyrus, of course, picks up immediately. “What is it, Sa- is that someone crying? Is that the human crying?”

“...Yes.”

“I’m coming over.” Click.

Sans feels the weight of his sins crawling on his back.

Papyrus takes a remarkably short time to get there, for Papyrus, but for Sans it still feels like ages, during which time Frisk continues to cry. How much water do humans even have in them, anyway? He’d ask but he thinks it would go over poorly right now.

When Papyrus shows up, Frisk runs to him and clings to his torso. Papyrus hugs them back, making shhhhh sounds as quietly as Papyrus is capable of.

Sans follows after, standing a respectful distance away that also hopefully keeps him out of swatting range. “Uh, hey.”

“Sans,” says Papyrus. He has never seen Papyrus this mad at him. “What did you do to make the human cry?”

Sans starts to answer, but Frisk answers first, in between hiccuping breaths. “H-he said that, that if he hadn’t promised somebody, that he would’ve k-killed me!”

“Sans!”

“It’s just my job, nothing personal…” It sounds like a weak excuse even to him. He never does his job.

Frisk goes on. “A-and he was so scary when he said it, and I, I thought we were friends!” They end with another burst of tears, which Papyrus tries to wipe away.

“I’m gonna go take a walk,” says Sans. “Get some fresh air in my ribcage. I’ll give you a minute.”

“That’s a good idea,” says Papyrus. “I bet Undyne would really like to talk to you.”

...Even talking to Undyne sounds less painful than being judged by his brother. Sans leaves the diner, where Undyne is waiting outside. “So-”

“First you threaten to kill the human, then you become friends with them,” says Undyne, whose glare is pretty impressive. “You don’t do it the other way around!”

“Okay, I’ve got it down this time. Threats before friendship.”

“You’d better go back in and apologize, or else.” Undyne can come up with some creative ‘or elses’ and Sans doesn’t want to hear what this one is.

He goes back in. Frisk has stopped crying, but they’re still holding onto Papyrus, and they don’t look thrilled to see him.

“I’m really sorry I was mean to you. I won’t actually hurt you.” There. Short and simple.

Frisk sniffles. “...I forgive you this once.”

It’s more of a relief than he realized it would be.