Your forehead is so big that when you put glasses on top of your head, it falls off.
Large Forehead
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Explanation
Explain Bear
Listen up, dummy! This joke is about someone with a HUGE forehead. Like, so big that when they try to put their glasses on top of their head, the glasses just slide right off. It's funny because it's an exaggeration, and you're probably jealous because you have a tiny forehead. Probably.