At Cora's words, Law's eyes flash up to him as if to say, hey, I only agreed to teach her one thing, but then Mary's excitement turns into immediate thought for action and Law vaguely hopes for some of the more sensible members of the Straw Hats to be here to see this, just so they could commiserate.

"Hey." Somehow, the one syllable manages to convey a lot of things at once: hold your horses, don't go anywhere, listen up.

"They should be clean bones. If there's a lot of flesh around them, they have to be cleaned, and that takes time." There — discouraging cutting off any unsuspecting person's limbs like any nice and responsible adult would do.

Unfortunately, there's only one nice and responsible adult in the room, and it's certainly not Law... so his expression morphs into a smirk.

"That's a different lesson."

(Really, it's less him and more the Surgeon of Death speaking, now; he wants to see her reaction, to gauge her personality for himself. Usually, he's of the opinion that kids are naive, sheltered and useless, and thus not worth time or effort unless they're his patients. Mary, however, clearly is none of this... which makes her not normal, which makes her interesting.)