Periscopes, including this one, can help you see around barriers. But what if you want to see something behind you? Submarine periscope operators walk in circles, turning the whole periscope. Why not just rotate the top mirror? Try it with this rotating periscope, and you'll see that the image of everything behind you is upside-down! An elegant demonstration of basic plane mirror reflection, this is also a great way to test student understanding of image formation.