I think if the center of the earth froze, it would be pretty hard core.
Hard Core Earth
Explanation
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.