How many prongs are on the fork?
Impossible objects work because our brain is trying to make sense of the situation. When we look at an object on paper, it is instantly interpreted and thought of as a 3-D object. This because your brain doesn't want to view a 2-D object because we see the world in 3-D. On the image above, your brain fills in the middle prong because it is looking at the object in 3-D even though it was drawn in 2-D.
"Impossible Object." Wikipedia. Wikimedia Foundation, n.d. Web. 18 Mar. 2015.
"BrainDen.com - Impossible Objects." BrainDen.com. N.p., n.d. Web. 18 Mar. 2015.
"Fooling Your Senses:." Impossibles. N.p., n.d. Web. 18 Mar. 2015.