"Nothing is impossible," Claudia said. She sounded to me exactly like a bad actress in a bad play—unreal. (9.253)
Finally achieving her goal makes Claudia an optimist. It also brings out her usual big headedness. We mean, of course she can tell Mrs. Frankweiler what's up in terms of life lessons. After all, she's a twelve-year-old girl and Mrs. Frankweiler is an octogenarian. Totally valid.