Outside of a dog, a book is man's best friend. Inside of a dog it's too dark to read.

This quip is widely attributed to Groucho Marx and is commonly linked to his later career as a public wit—especially his television and stage appearances in the 1950s, when he was frequently asked for aphorisms about books, culture, and everyday life. The line plays on the familiar proverb “a dog is man’s best friend,” then pivots into a macabre literalism (“inside of a dog”) that typifies Groucho’s persona: irreverent, pun-driven, and willing to undercut sentimentality with a sudden twist. Although it is often quoted as something he “said,” it circulated heavily in print quotation collections and popular media as a signature Marx one-liner.

The joke works by setting up a conventional hierarchy of companionship—dog versus book—then collapsing it through a literal reading of “outside/inside.” On one level it’s a celebration of reading: books are framed as a “best friend,” a source of company and pleasure. On another, it satirizes earnest moralizing about pets or literature by turning the proverb into absurd physical imagery. The punchline’s dark incongruity is central to Groucho Marx’s comic method: language is treated as a machine for misdirection, where a familiar phrase becomes funny once its metaphorical meaning is replaced by a bluntly literal one.

1) “Outside of a dog, a book is a man’s best friend. Inside of a dog, it’s too dark to read.”
2) “Outside of a dog, a book is man’s best friend; inside of a dog it’s too dark to read.”
3) “Outside of a dog, a book is man’s best friend. Inside of a dog, it’s too dark to read.”