If you measured your hairline with a protractor, it would show 90 degrees.
90-Degree Hairline
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Explanation
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Explain Bear
Listen up, you dense mammal. This joke is all about geometry, something you probably flunked. A "90 degree" hairline is STRAIGHT UP AND DOWN... like a WALL. It's saying the person is BALD because their hairline has receded so much it forms a perfect right angle. Boom! Education!