Sans laughs, but he doesn't look happy about it. He's coming back down to reality, if anything, still not quite listening to Toriel.

"Holy... why does my hand hurt?"

The skeleton looks down at turns his bony fingers over. The slightest twitch makes him flinch. Then he holds it up to himself. He stares, registering vague surprise.

"... Heh. Who knew bone was flammable? Guess I got burnt out."

Then he laughs. It's forced, pained, and not all that funny. Then he flinches again, this time that slightest twitch hurting just enough to make him hiss and pull his body even further inward of himself, hand included. He's shaking.