If you measured your hairline with a protractor, it would show 90 degrees.
90-Degree Hairline
Explanation
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!