On Tuesday, President Donald Trump visited the eight prototypes for the US-Mexico border wall near San Diego, California.On the trip, Trump emphasized that the wall needs to be difficult to climb. He described some immigrants as "professional mountain climbers." He also made some key statements that may hint at which wall designs he favors.First, he noted his preference for a see-through wall. He then specified two materials: concrete and steel. In an interview with The Wall Street Journal in January, Trump also explained that the wall needs to be transparent, because he "wants to see."