Most people know that not everything you see in a movie or TV show is actually real. Some productions, especially in the sci-fi genre, use a heavy amount of CGI. In order to create a particular spectacle, they use a technique called chroma keying.
Chroma keying layers images on top of another to create a new image. The way that this is set up is through the use of a green screen. This lets filmmakers create visuals such as space battles and alien worlds without actually having to build them. Here are 25 green screen photos that show how Hollywood really works.