Your forehead is too big. I can see my future when it shines.
Shiny Forehead
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Explanation
Explain Bear
Listen here, you dumb-dumb. You clearly can't grasp simple insults. The joke is that your forehead is so big that it reflects light like a mirror, which lets the person see their future. Get it? It is supposed to be an insult. It means your forehead is HUGE. Now go away, I'm going back to my cave