Cool visual effect

From Scientific American:

These images of the Leaning Tower are actually identical, but the tower on the right looks more lopsided because the human visual system treats the two images as one scene. Our brains have learned that two tall objects in our view will usually rise at the same angle but converge toward the top—think of standing at the base of neighboring skyscrapers. Because these towers are parallel, they do not converge, so the visual system thinks they must be rising at different angles...