What Is the "brown Chicken, Brown Cow" Joke?

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The "brown chicken, brown cow" joke is a joke consisting of a sexy question related to farming and followed by the punch line "brown chicken, brown cow." For example, one version of the joke asks, "What are the sexiest animals on a farm?" And the punch line is "brown chicken, brown cow." This is meant to be funny as "brown chicken, brown cow" sounds like "bow chicka bow wow."