Heck, he had no thought about building a wall and it was never serious- he can't keep talking sense without going all over the place so his advisors figured three words would keep him focused: "Build a wall"

Trump is an intellectual lightweight and shallow thinker. I doubt he ever had any idea of mineral deposits in Greenland much less knew where Greenland was! Someone mentioned it to him his last term and he keeps shaking it like a dog shakes a squirrel.