Take a picture, and... ah, ok, yeah, guess Sans should've seen that coming. The reference sails on a graceful arc far over this guy's head, and that's just one more ounce added to the scales on which Sans measures the likelihood that they're even from the same world.

"Eh," Sans remarks, winking at the young man, "Fuhgeddaboudit."

And then lets the reference drop. Once you have to explain a joke, it ceases being a joke. Sans won't judge the guy for not getting it, but this now leaves him with the question of whether or not he should give the guy a straight answer.

In the end, Sans doesn't see any harm in it.

"Easy, pal," goes to show that cooperation need not mean total acquiescence. There's a chuckle in the low rumble of the skeleton's voice, as he leans easily against the rail. "I'm gettin' to it. I'm from the Underground, on a planet called Earth."

He continues, flicking phalanges vaguely at the drop ahead of him, "Don't sweat it if you haven't heard of it, or even if you have, you wouldn't have heard of the Underground. Humans ain't so big on forgivin', but forgetting?"

He rolls his eye lights, grinning like that's funny.

"Heh, they're regular professionals."