One twin tower had a girlfriend. The other twin tower had the same girlfriend, so they both went down.
Twin Towers' Shared Girlfriend
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Explanation
Explain Bear
Listen up, buttercup. This joke references the Twin Towers (aka the World Trade Center) that were attacked on 9/11. "Going down" is used in the joke to imply both having sex with the girlfriend and the towers collapsing, you walnut.