Are these giant teacups about to tumble down? No, it's an optical illusion—something that looks different than it really is. Artist Leon Keer painted four 25-foot-tall teacups on a building in Helsingborg, Sweden. Although the image looks 3-D, it's actually a flat mural. But when viewers look at it using a special app on a smartphone, the teacups appear to fall to the ground.