Gotcha! But the rest of these optical illusions are on the level. What are illusions? Illusions trick us into perceiving something differently than it actually exists, so what we see does not correspond to physical reality. Hence, the word illusion comes from the Latin verb illudere meaning, "to mock." In addition, some illusions show us one thing in a picture, while someone else sees something entirely different in the same picture. Research scientists must be sure that the results of their work are not "illusory" in nature.
Related: `test 1028