An interesting thing that I recently came across was something called an impossible figure. An impossible figure is an illusion created by a two dimensional figure that the visual system subconsciously interprets as a three dimensional figure. At first, the brain processes the figure to be three dimensional but after staring at the figure for a few seconds you begin to realize that it is impossible and that figure is really two dimensional.One person who mastered this illusion was M.C. Escher. In his famous drawing Waterfall (shown below), the path that the stream of water takes appears to recede farther away rather than go to the top of the waterfall. He uses cues to enhance the height in field effect which makes the water appear to flow downhill and still end up higher than it started.
Another famous impossible figure is the Penrose triangle(above), which appears to lie in a plane and one side seems to project out at an angle from the other two sides. Both of these images appear this way because of Gestalts laws of closure, which state that we will see objects as wholes and fill in what is missing. Here are a few more pictures below, can you see how they are impossible?