For some reason, that joke isn't just unfunny anymore, it's... offensive, he thinks that's offense crawling up his bowed thoracic vertebrae like hot acid. Sans is irritated, not at the kid so much as at his own illogical reaction. In a show of the usual self-destructive negligence, Sans doesn't bother varnishing his opinion.

"There's two things I don't do, kid. Promises and reruns," he says, shrugging even as he kills the joke, "Call it a personal preference. Sorry, pal."

No amount of winks and comically insincere apologies can exactly take the sharp edges off his refusal. It's, uh, kinda mean, actually, and the fact that Sans feels bad about that just means he doesn't do anything about it. He told the kid before, didn't he? There's better people to hang with than him.

"So...

"...Whaddya think is so important, valuable, or dangerous that they gotta lock it up behind a door like this?"