A: By the way, if you want to know if a wall works, just ask Israel. Israel built a wall and it works.

Q: And they heave rockets over it.

A: Yeah, I know. Well, no. Now they’re doing the rockets, yeah. That’s a — they have a — they have a different — they probably have a bigger — they have a different kind of a problem. You have to build a real wall. They don’t have a real wall right now. They don’t have a wall that works.

• • •

Did you get that?

1) There’s a wall in Israel and it works.

2) But it doesn’t work.

If you are confused, it’s probably your fault, because you are not Donald Trump, master builder, president of the United States and source of the quotation above, from an interview with the New York Times’ editorial board before the election.

Trump has always said stuff like this, things that are self-contradictory or untrue or breathtakingly mindless. It didn’t matter so much back when he was just a rich guy who liked to share his opinions with the world the way some people talk at the TV.

But now he is in the Oval Office, and the stuff he says is treated differently. A lot of it blows away, but some stuff actually happens. Things roost in his brain and come out of his mouth and Twitter feed, and before you know it, the federal government is taking proposals for Trump’s great border wall with Mexico. Hundreds of companies are expressing interest, preparing designs, creating renderings. Finalists are to be announced in June. Prototypes will then be built. It seems certain that millions or billions of dollars will be wasted, and miles of desert despoiled, before somebody someday pulls the plug.

The man in charge of the wall for now is John Kelly, the secretary of homeland security. Kelly does not think a wall alone is the best way to secure the homeland. He said at his confirmation hearing that a “physical barrier in and of itself will not do the job.” He added, “I believe the defense of the southwest border really starts about 1,500 miles south” — that is, in helping Central America tackle the root causes of drug smuggling and migration. Nor does he think that erecting a wall as Trump describes it is even possible. Speaking to the Senate Homeland Security Committee last week, he said, “It’s unlikely that we will build a wall or physical barrier from sea to shining sea.”

But Kelly is no longer an independent retired general. He is now on Team Trump, and no matter what obstacles are imposed by reality — by topography, by physics, by Congress and by the budget process — a big, beautiful wall is what the boss wants.

Customs and Border Protection specifications say the wall must be concrete or some other material, preferably about 30 feet high but no less than 18 feet, “physically imposing,” “aesthetically pleasing,” impervious to tunneling to a depth of 6 feet and tough enough to repel a sledgehammer, pickax, acetylene torch or similar tool for at least an hour.

Those are tough requirements for a project whose very rationale collapses under the pressure of a few minutes’ thought. The libertarians at Reason magazine have a fine summation of why the wall won’t work. If only Trump would read it, or ponder these questions:

How do you build a wall along the 1,200 miles of the Rio Grande, the Texas stretch of border? Do you put it on our side and abandon the river to Mexico, or seize Mexican territory for it, or put it in the middle of the river, or do some zigzag compromise? What do you do then about a treaty requiring that both countries have open access to the river?

How do you make a concrete wall see-through, so smugglers aren’t invisible to the Border Patrol?

How do you get private landowners to go along? What about the Tohono O’odham Indians, whose reservation straddles the border in Arizona and who want no part of any wall on their sacred land?

What happens when parts of the great wall become a great dam, trapping floodwaters and debris, and collapse?

How do you wall out deep tunnels, drones and catapults? What about the tons of drugs that pass through existing ports? Did you know that drug cartels have ships and submarines? What happens when drug bales start coming ashore in San Diego, or over from Saskatchewan?

Not that you care, but how do you build a wall that doesn’t scar fragile wilderness and destroy wildlife?

How do you avoid causing more human deaths when a partly walled border sends migrants to more remote and deadly stretches of desert?

Do you realize that a wall would trap millions of unauthorized immigrants in the U.S.? Or that a large percentage of them come here legally, through ports of entry, with valid visas, and never go near the border?

It’s too bad these and other questions weren’t considered before the wall began taking shape. It has been a remarkable journey for an idiotic idea that started life as an applause line, as Trump admitted to the Times.

“If my speeches ever get a little off,” he said, “I just go: ‘We will build a wall!’ You know, if it gets a little boring, if I see people starting to sort of maybe thinking about leaving — I can sort of tell the audience — I just say, ‘We will build the wall,’ and they go nuts. ‘And Mexico will pay for the wall!’ But — ah, but I mean it. But I mean it.”

In other words:

1) The wall is a decoy, a fake, a lie.

2) But he means it.

Did you get that?