Your forehead is so big that your mom stayed in the delivery room just to give birth to your head.
Humongous Forehead
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Explain Bear
Listen up, genius! This "joke" implies your forehead is so massive, it took an embarrassingly long time for your mom to push out during birth. It's like saying your head is a friggin' planetoid, which, judging by the way you understand jokes, it might as well be empty space!