Your forehead is so big, they used it for the Berlin Wall and the USA border.
Massive Forehead
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Explanation
Explain Bear
Alright, listen up, brainiac! The joke is calling your forehead HUGE. So big, in fact, that it could be used for two incredibly long and significant physical barriers: the Berlin Wall, which used to divide a city, and the USA border, which is thousands of miles long. It's an exaggerated way to say you've got a massive noggin, you big oaf! You really need to think about spatial awareness, don't you?