That actually drew a laugh out of him, caught between one Frisk would recognize and one from a timeline they blessedly never saw.

"Kid, that's a big question."

For a long time, Sans wished he could hold onto his memories between resets. At least then progress could be made, on the machine or even just understanding why the child did what they did. In the light of recent experience, Sans wanted to laugh at those old misguided wishes. Losing his memories with each reset was one of the greatest gifts he was allowed in his sad excuse for a life.

"I think I missed my cue. This is the part where I shake your hand, right?" He turned back around, rubbing the heel of his palm against his eye socket with a broken chuckle. "Or maybe this is the part where my bro tries his hardest to get you to play along. I gotta tell you kiddo, I'm lost, so maybe you could fill me in?"

There's a sharpness to his voice that the Sans from Frisk's timeline would never use. Maybe towards school bullies or the few humans that didn't care for monsters walking among them, but not on Frisk. Never them.