[There is more to explain, and better ways Sans could explain it. But his already careening train of thought falls off the track entirely when he absorbs what Papyrus says. He stares, eyelights locked on his brother.]
What?
[He knows what Papyrus means. But it's never once occurred to him.]
no subject
What?
[He knows what Papyrus means. But it's never once occurred to him.]