Science behind it
One theory is rapid eye movements. Your eyes dart back and forth faster and faster and this changing view creates a continuous stream of visual images.
"These Optical Illusions Trick Your Brain With Science | WIRED." Wired.com. Conde Nast Digital, n.d. Web. 17 Mar. 2015.
DiSpezio, Michael A. Eye-popping Optical Illusions. New York: Sterling Pub., 2001. Print.
"Optical Illusions Mess With Your Mind." HubPages. N.p., n.d. Web. 17 Mar. 2015.