Billie hooks the mask back to her belt and gives Strange a funny look when he jumps right to the magic bullshit part of the rescue. Admittedly, it was her magic bullshit and she likes it very much, but still, there's something irksome about him assuming she's 'self-taught'.

So rather than take it as a compliment, she takes it as a reason to get a little sarcastic, her reply coming out deadpan with, "What? You're so sure someone like me couldn't come from one of your fancy magic schools?"