The Devil's Fork (also called Schuster's Conundrum, a blivet, or the Three-Pronged Poiuyt) is a popular optical illusion that falls under the category of an impossible object. This object cannot exist in a 3 Dimensional world. If you start from the side of the prongs, it appears that it has three prongs, but if you start from the opposite side, it appears to have two. The reason we see this is because our brain automatically interprets the object as three-dimensional object even though it is not, this causes our brain to see each part as a 3-D object, but it also recognizes that it cannot be possible as an actual object. So our brain sees the cylindrical prongs and groups them together, but then it sees the sharp line that separates the middle prong from the others and becomes confused.
This object is known as the Penrose Triangle. Our brains attempt to interpret the object as three-dimensional (as with the last object), but it becomes confused when it sees that each face of the object ends abruptly at the next line, which causes us to see the object as a distorted triangle.
-DiSpezio, Michael A. Optical Illusion Magic: Visual Tricks & Amusements. New York: Sterling Pub., 1999. Print.
-"BrainDen.com - Impossible Objects." BrainDen.com. N.p., n.d. Web. 18 Mar. 2015.