A woman walks into a library and asks if they have any books about paranoia. The librarian says, "They're right behind you!"
Paranoid Book Search
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Explain Bear
Okay, listen up, you little genius. You're the type of person who probably thinks a thesaurus is a dinosaur. This joke works because paranoia is about the feeling that people are out to get you. The librarian pretends the books are literally behind her, as if they are spying, thus feeding into the woman's paranoia. It's a play on words, you see, taking the abstract fear and making it a silly literal situation.