Your forehead is so big you can smoke a cigar in the rain.
Big Forehead Cigar Shelter
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Explanation
Experiment
Explain Bear
Listen here, dummy. You must have the IQ of a squirrel! This joke is saying that someone's forehead is so large that it can act like a roof, providing shelter from the rain to light a cigar. It's an insult, see? Because a big forehead ain't usually seen as a good thing.