Sans closes his eyes, after a while - he's still pretty tired from transforming in the first place. That sort of ache doesn't easily fade. And he's just considering trying for a nap right here when Frisk speaks again.

Even so, Sans doesn't immediately give any sign of having heard them. Even now, he can appreciate that that won't be an easy question to answer. Not even if he lies.

In the end, he decides to go for technical honesty. "Not at first." But there was an accident.