In the same way you know the sky is blue and gravity keeps things from flying off into space, you probably know that Donald Trump’s plan to build a gigantic wall on the U.S.-Mexico border is a bad idea. But in case you wanted to know, specifically, exactly how bad of an idea it is, John Oliver devoted a lengthy segment on his show Sunday night to doing what even Mr. Gorbachev never could: Tearing down a wall before it could ever be built. As Oliver explains, not only would the wall be incredibly expensive to build and maintain, it would also be remarkably easy to circumvent. Oh, and its construction would involve expropriating private property and disturbing Native American burial grounds. In nearly every way that an idea can be dumb, this is a dumb idea.