"...if you can't see where it keeps its brain."
Wise, all-purpose advice from J. K. Rowling's Harry Potter and the Chamber of Secrets. Sound advice for Muggles and Wizards alike. (Quote appears early in the final chapter, said by Mr. Weasley to one of his children... no spoilers if you haven't read the book).
Phrase resurfaces at least once more when Harry Potter receives the Marauders' Map in Harry Potter and the Prisoner of Azkaban.