[Sans has to think about that, because while the knowledge has grown to swallow up so much of his life, he didn't always know it, and he didn't learn it all at once. He scrubs at his eye sockets, which only helps so much.]

It was related to what we were working on in the lab, but that was theory. Math and observations of things that said it might be possible, not that it was happening. And then when dad died... [The details are still swimmy around that time.] And I couldn't fix the machine, I figured--well, that's that. But then--it'd be about four years ago, now, but it was two years ago from when I last remember being home--I started noticing things seeming familiar--seeming real familiar. And this isn't the first timeline, even if I don't remember, so I just knew--things that leave strong impressions, right?--I knew I had to start looking into it. I don't know what left a big enough impression on some other Sans to start it off, but it must've been around the same time.