I think if the center of the earth froze, it would be pretty hard core.
Hard Core Earth
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Explain Bear
Alright, listen up, buttercup. This "joke" is all about wordplay, see? "Hard core" usually means tough or extreme, but here, it's also a pun because if the Earth's core froze, it would literally be hard. And a core. Geddit? Probably not, you walnut.