triangle fill in illusion
your eyes see a triangle so it flips it in the blank space or something of the other like that, also the black circles and lines make your mind think that they are shadows, those shadows of a.. get this, triangle.
N.p., Web. 17 Mar. 2015. <https%3A%2F%2Fencrypted-tbn1.gstatic.com%2Fimages%3Fq%3Dtbn%3AANd9GcSMgVb8m46F1lSDb4H3zugdFDBYurrkLWpUJyNz6H6eB2Hq7Rbn>.