What is frayed knot joke?

The "frayed knot" joke is a play on words, with the punchline sounding like "free not." The joke typically goes something like this: "Hey, can you tie a knot?" "Sure, why?" "Just wondering if you can tie a frayed knot." The punchline is then delivered with a sarcastic tone, implying that the person is incapable of tying a knot that is frayed. The joke is often used to poke fun at someone's perceived lack of expertise in a particular area.