People say that when you see an image it's displayed upside down and your brain flips it to be right side up. Well, it turns out that this little factoid can actually matter. Get a flashlight (torch) with a pretty good amount of light output and place it on your forehead. Close your eyes if need be. You'll see a bright red glow coming from what looks like down to you. Because you are getting light to your eyes that doesn't go through your lenses. The reason it looks like it's down, is because it's really up and you're use to everything being flipped.