[Sans disagrees with her assessment that he's cool, but instead he just keeps talking about Papyrus.] Yeah, we made that outfit for a costume party, but he liked it so much he kept wearing it. Calls it his Battle Body.

[Then Sans realizes. It's not so bad, since it's Sayori and Sans likes talking about Papyrus anyway, but it's still unsettling.] It's happening again. This place is getting in our heads.

[Sans turns away from the mirror in a deliberate motion, so he doesn't see the reflection change. Papyrus vanishes, replaced with a younger Sans dressed up like a scientist and watching Sans and Sayori both with a blank expression, even by skeleton terms.]