He freezes. He wasn't moving in the first place, so it might not be noticeable, but he feels his heart skip a bit. The quiet that rings out after Sans words feels deafening. The ending had been the same, and it seemed, maybe so had the beginning.

"They said... they were going to fix me." he tries to laugh at the joke of the statement it is, but the statement makes his heart turn and the laugh dies in his throat.