Gunnar Johansson's `walking lights' animation
This is a well studied classic in experimental psychology and is one of the most robust perceptual phenomena known. Twelve moving dots in a two-dimensional plane are unconsciously assumed to represent a particular three-dimensional motion. When the dots are moving, everyone with normal vision sees a person walking. This is central to the arguments in my new book Science, Music, and Mathematics: The Deepest Connections.
Animation by kind courtesy of Professor James Maas.