The Jastrow Illusion is an optical illusion discovered by Joseph Jastrow in 1889.  The illusion works by messing with the perception.  While people are not entirely certain how this illusion works, the accepted hypothesis is that people compare the short edge of the upper figure to the long edge of the lower one, or if flipped (which causes the lower figure to look larger) the long edge of the upper figure would be compared to the short edge of the lower figure.  This contrast between two edges placed together tricks the mind into perceiving one figure as larger than the other despite the two being the same size.