You have to be able to see through it.
The White House originally said Mr Trump's comments on Thursday were off the record, but later changed course and released his remarks. "In other words, if you can't see through that wall - so it could be a steel wall with openings, but you have to have openings because you have to see what's on the other side of the wall", Trump stated. According to the President, despite the craziness of an idea of the glass wall, it would need to be see-through to stop passersby being hit with heavy sacks of drugs being thrown over it.
But on his way to Paris, Trump told reporters aboard Air Force One, "It's a 2,000 (3,218 kilometer) mile border, but you don't need 2,000 miles of wall because you have a lot of natural barriers".
The President added "violent and vicious" rivers provided a natural barrier along parts of the border.
The winding Rio Grande defines the frontier in most of Texas, though in parts it is little more than a trickle of water, while the Colorado River marks the boundary along 24 miles in Arizona.
Picking up on the theme laid out by Homeland Security Secretary John Kelly and other border experts and agents, Trump spoke about the need for any border barrier to allow agents to see the other side. "It's over", Trump said.
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Trump also acknowledged Thursday that "violent and vicious" rivers and natural barriers along the southern border likely mean that the wall will not cover the full stretch, which is almost 2,000 miles. They hit you on the head with 60 pounds (27 kilograms) of stuff? You have some areas that are so far away that you don't really have people crossing.
Andres Rozental, Mexico's former deputy foreign minister and ambassador to the United Kingdom, told Sky News the proposed see-through wall was "the most ridiculous thing I have heard in many, many years".
He added: "It just doesn't make any sense".
As a presidential candidate, Trump campaigned on the promise to build a massive wall along the U.S. -Mexico border - a project he says Mexico will pay for.
He has since said he will find a way for Mexico to repay the USA for construction of the wall but that Congress would need to fund it first.