This simple yet confounding model was named the Illusion of the Year by the Meiji Institute for Advanced Study of Mathematical Sciences, Japan. Watch the clip, allowing yourself the thrill of confusion before seeing how it works 25 seconds in.
And when you're done with that, you can build your own, as the inventor shared his plans for the device on his website...along with his tale of how he discovered the illusion:
This kind of optical illusion, which I named "Impossible Motion", was discovered in my engineering research for machine interpretation of line drawings. In order to check the behavior of my software, I gave what are called "pictures of impossible objects" as the input expecting the software to judge them impossible. However, my software sometimes interprets "pictures of impossible objects" as actual solids, from which I found that some of "pictures of impossible objects" are not impossible; they can be constructed although general angles are used where they look rectangular.
Wait a second...was this illusion made by a man or a computer—and who got the trophy and giant check? I demand a recount! (while I sob into my pillow that neither my computer nor computer operator is so smart) [Impossible Motions and Illusion of the Year Contest via Discover via Neatorama]