Resets come in a lot of different forms. It's incredibly easy to kind of see them everywhere, once you have that sort of framework. Every setback is a Reset. Every time someone leaves Wonderland, it's a Reset. When he eventually leaves and forgets everything, that too will be a Reset.

Some things don't Reset all the way. Some things you can't come back from.

Mettaton mentions the kids digging and...that's such a specific way to put it that Sans glances at him sidelong. Did one of the kids say something to him? Bring up Gaster, maybe, or something similar?

"no, it...that was a real long time before the kids were in the picture."

The kids are always digging where they aren't supposed to, but that isn't the issue this time.

"years and years ago. papyrus just kinda...made assumptions about what happened. and i never bothered to explain. eventually, the..."

He grits his teeth for a moment, pausing.

"eventually, the uh, the person who--he kinda--stopped being a factor."

Boy, what a way to put it. What a way to sum up the total erasure of an entire person. Sans shrugs.

"and papyrus just stopped thinking about it."

There were a lot of things people stopped thinking about; a lot of blanks that people never bothered to fill, because they could never be filled. No one thought too hard about who had built the Core. It was just there. It was there and it worked, and in the grand scheme of things, that was all that mattered.

Mettaton moves on anyway, touching on that old doubt, the old feeling of not the right Sans. Two Papyrus-es, and Sans was always the uncommon denominator. The odd one out. The wrong one. But Mettaton has a point, the same point Papyrus was always making--it doesn't matter. While you're in Wonderland, you belong. You belong to each other.

"heh." His grin turns openly wistful. "he thought the same way. no wonder you two got on so well."