He can tell from the tone in which she repeats the sentence that it was not said to her kindly.
"Okay. We've definitely got a fairly major - cultural gap," this is actually the LEAST CONCERNING POSSIBILITY here, "because in this world even people who are very very strong, whose pride and will would not be questioned by any sane person, like Creation mages, or nobles who've held up under torture for months and come out angry and successfully vengeful, will not try to motivate themselves to channel Acid by putting their own feet in acid, especially if they're not sure they've even got that channeling capacity."