Over the weekend, Donald Trump, who continues to insist that Americans pay for a giant wall along the U.S./Mexico border, said that the Mexican government will eventually “reimburse” us for the cost of the project. Mexico’s Foreign Ministry said soon after, “Our country will not pay, under any circumstances, for a wall.”

Which, naturally, led the American president to again say yesterday, “One way or the other, Mexico is going to pay for the wall…. We may fund it through the United States, but ultimately, Mexico will pay for the wall.” I assume Mexican officials will respond in kind again soon.

But while the back and forth is tiresome, especially about a project Congress is highly unlikely to approve, Trump also explained why, exactly, he thinks the wall is so necessary.

“We need the wall very badly. As you know, Mexico has a tremendous crime problem – tremendous – one of the number two or three in the world. And that’s another reason we need it. And the – just to add on, tremendous drugs are pouring into the United States at levels that nobody has ever seen before. This happened over the last three to four years in particular. The wall will stop much of the drugs from pouring into this country and poisoning our youth.”

No, it won’t. Perhaps the president hasn’t had time to read up on his signature issue, but there’s no reason to believe the wall would serve as some kind of anti-drug barrier. “A wall alone cannot stop the flow of drugs into the United States,” Christopher Wilson, the deputy director of the Mexico Institute at the Wilson Center, recently told Vox.

A Washington Post report added, “[E]xperts on the drug trade say a border wall, even one as big and beautiful as Trump promised, would be near-impotent in stemming the supply of illegal drugs.”

In other words, the president’s top rationale for building a wall is based largely on his confusion about what a wall would do. But at the same press conference yesterday, Trump also went just a little further:

“As you know, I have [Retired Gen. John Kelly, the White House chief of staff] here. We stopped traffic coming through – 78 percent. It’s going to be, I think, 81 percent this quarter, which is a record. In other administrations, if they stopped it just a little bit, like one, or two, or three percent, they considered that a great thing. We’re up to almost 80 percent. We’ll soon be over 80 percent.”

There are two important problems with this. The first is the apparent contradiction: Trump wants us to believe there’s a crisis at the border that only a wall can solve, and at the same time, he also wants us to believe he and his team have already dramatically curtailed illegal border crossings.

The second is that Trump keeps throwing around that “78 percent” figure, despite the fact that it’s a made-up statistic that plainly isn’t true. The president somehow got that number in his head, but every independent analysis has made clear that it’s wrong. USA Today reported last month, “We don’t know how the president arrived at 78 percent. We repeatedly asked the White House for the president’s source, but we received no response.”

Trump likes to manufacture details that make him happy, but that doesn’t make them true.