Chroma keying allows media technicians to easily separate green screens and panels from the people standing in front of them and replace those backgrounds with pretty much anything — from animated weather maps to the skyline of 1930s-era New York City to the icy Wall guarded by the Night's Watch in HBO's hit TV series "Game of Thrones." [Science Fact or Fiction? The Plausibility of 10 Sci-Fi Concepts]