They say people can have a sharp mind. Yours is like a dull knitting needle.
Sharp Mind, Dull Needle
Explanation
Explain Bear
Listen up, you big brain, or lack thereof! This joke is basically saying someone isn't very smart. A 'sharp mind' means you're clever, but a 'dull knitting needle' means you're about as sharp as a spoon. It's an insult, you big dummy.