U.S. President Donald Trump's administration is once again touting black paint on border walls to deter illegal migration — a questionable and expensive proposition.

Homeland Security Secretary Kristi Noem said the directive came directly from President Trump, under the expectation the dark paint will discourage migrants to climb the wall by making it unbearably hot under the desert sun.

The wall along the southern border will be painted black in an effort to make the structure too hot to climb, Department of Homeland Security Secretary Kristi Noem announced Tuesday, saying the suggestion came from President Donald Trump.