"it...it might not be a matter of me getting hurt, s-4." At least, not in a way that his brother could heal. "it might be a matter of me...hurting other people." Especially Frisk. The thought of that happening, so soon after he'd thought that Frisk was gone forever, makes Sans feel physically sick.

But that's why S-4 has to know. That's why a few others have to know, too, but it's easiest to start with his brother.

"i told you a little bit about how i got my powers, r-right? that gaster was my brother, and the two of us worked together to try and find a way to stop the anomaly? well, i mean, it...it basically worked. as well as could be expected. eventually. but, uh...not without some problems." He winces at how much of an understatement that is, but Sans' memories of that time are still quite hazy, so what else can he do?

"over the past few days, i, um...i tried to replicate some of the things gaster did to me. to try and make myself even stronger, y'know? even though that's, that's what made me change into that thing in the first place. i thought i fixed where we went wrong. for a while, i was sure i did. but..." He lifts his previously-wounded arm, staring pensively at the neatly-healed bones. It should have healed without S-4's help. It hadn't.

"...now, i don't know. it might be nothing. it might be okay. but in case it's not...you should know."