I agree with Faldage. When I was little I would get "optical illusion" books from the library, to amuse myself. And they would have drawings in them as described, where you see two images at once. There are famous ones: the vase/faces one, and the young lady/old lady one. The Canadian flag is such an illusion too. If you look for an image in the white space you see two angry faces looking kind of downward - they say it's symbolic of the everlasting conflict between the French and the English - the two solitudes!