A picture represents a 3D object. Since the picture is flat it doesn't have follow the laws of 3D. So you can draw things that twist,turn and do anything then our mind will make it look like it's 3D.
"Elephant Legs." Optics For Kids. N.p., n.d. Web. 18 Mar. 2015.
DiSpezio, Michael A. Optical Illusion Magic: Visual Tricks & Amusements. New York: Sterling Pub., 1999. Print.