They make a noise in response--mleh. Since Chara won't actually be able to see if they stick their tongue out.

"...that's not a...good theory. Notta explanation at all. Luck's not a thing. Not back home, anyway. I guess it might be here."

More rhythmic tapping. "Underground has to do with Determination. But I dunno how it works. How you can put it in...needles...and stuff." They're sort of assuming Chara made it down into the True Lab, too. Sans did say they killed everyone.