My sister borrowed my favorite book.

This joke hinges on a delightful bit of wordplay, taking a familiar phrase and twisting it into something absurd. The humor comes from the punchline, "return it by the chapter," which plays on the common idiom "return it by the day" or "by the week," implying a very strict deadline. Instead of a temporal measure, the joke uses a structural element of a book, creating an impossible and comical condition for its return. It's a clever subversion of expectations that makes you chuckle at the sheer impracticality.

The real-world context here is incredibly relatable for anyone who has lent out a prized possession, especially a beloved book. There's a universal anxiety about whether you'll ever see your treasured item again, or if it will come back in the same condition. Libraries, of course, have strict return policies, and people often set their own informal deadlines when lending things to friends (Review) and family. This joke taps into that feeling of wanting your favorite book back almost immediately, exaggerating the sentiment to a hilarious extreme by suggesting a return schedule that's literally chapter by chapter.