The Jastrow illusion is an optical illusion discovered by the American psychologist Joseph Jastrow in 1889. In this image, the two figures are identical, although the bottom one appears to be larger.
So you look at the above and say OK maybe they are the same size. You can even see that it is true. How about the photo below, is it possible that these two are the same size?
Even knowing that the two above are the same size it is hard to believe. Look at them how is it possible for them to be the same size? Well you don't have to take my word for it, check out the video below. This time Seeing is believing!
Filmed at the Hong Kong Science Museum by YouTube.com user kennsiu.