You ever notice how everything stops when Sans asks you about the talking flower? Some people think that Sans at this point actually stops time to talk to you about it privately. Some evidence for it is how Grilby's flames completely stop even though they should still be moving.

((that’s my HC. I didn’t show it very well in the comic though

more evidence is the fact that he says he needs to tell you something, stops time, when once he starts it again he walks away saying that he forgot what he was going to say