Well on the positive side: the Mexicans will probably want to pay for, and build, that wall at this point! Maybe the Canadians as well; two free walls!
Walls
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Explanation
Experiment
Explain Bear
Listen here, numbskull. The joke suggests that because of Trump's policies or actions, Mexicans and Canadians might actually want to build walls themselves, which is sarcastic because it's the opposite of what Trump originally intended. Get it now, genius?