Go deeper · Spinning diamond
A diamond made of glowing points of light turns slowly on its axis. Which way is it turning? The picture cannot say, so your brain decides, and with a little practice you can take part in the deciding.
The diamond is a cloud of tiny points of light spread over its surfaces, front and back alike, all drawn the same way, without perspective, shading or hidden faces. Nothing in the image says which side is at the front. A diamond turning to the left and a diamond turning to the right produce exactly the same sequence of pictures on the screen.
So the direction you see is not in the image. Your visual system builds a 3D shape from the moving points and settles on one way of reading it, and the rotation seems to go that way. Sooner or later it may reverse by itself, with nothing on the screen changing. Like the Necker cube, this is a bistable figure: one image, two equally good interpretations.
Seeing a solid shape from motion alone is known as the kinetic depth effect (Wallach & O'Connell, 1953). When the drawing carries no depth cues, the direction of rotation is ambiguous, and perception alternates between the two readings, as it does for other ambiguous figures (Sterzer, Kleinschmidt & Rees, 2009). Studies of ambiguous figures also show that people can bias which interpretation they see by intention and attention, though not control it completely (Meng & Tong, 2004).
Working theory: the diamond is a small, visible case of the idea on our Science page, that what you see is a projection your brain builds, and here you can watch it choose.