Light waves bounce off of its surface and reflect into your eyes. It becomes a 3D object and it changes direction. Your eyes try to see something but the light waves change what your eyes see. In this one when you look at it, it changes your view of the light waves.
Citations:Optical Illusion Facts and Information for Kids | KidsKonnect." KidsKonnect
"Optical Illusions | Eye Didn't Know That!" Eye Didnt Know That Optical
"Optical Illusions." Optical Illusions. N.p., n.d. Web. 19 Mar. 2015