Rosethorn's heart sinks when Papyrus gets to the word dust. Papyrus told her after that incident in the garden that there's nothing more to skeleton monsters than dust and magic. It's obvious what happened, and just as obvious that Papyrus is nowhere near ready to accept it.

She could kick herself, because she knew. She fully expected this to happen, but she handed the problem off to someone else rather than taking responsibility herself. If skeletons bled, she would have Sans' blood on her hands. And she had understood, after seeing how Sans reacted, exactly why Papyrus hadn't wanted his brother told.

"What room are you in, Papyrus? I'll be right there."