So it's true. Even something like this... it could be dangerous.
["Innocent crap", as he puts it, is hardly threatening on its own; but Sansa is unfamiliar with magic, and the thought of it doing... something else, the knowledge that it can change even something like her clothes at will so easily... it isn't reassuring.]
I understand. But... what good is that, if we can't stop it? Even if we're careful, if they can do something like this so easily... isn't there anything to stop that?