The box’s outline: it never leaves it.
Tricks of motion
A box that goes round but stays
Follow the arrow. Does the box go where it points?
It seems to: right, down, left and up, round and round. But the box never leaves its place.
Why? The box only fades lighter and darker while two of its edges, one dark and one light, swap sides. Your eye reads each change as a step. Swap the left and right edges and it steps sideways; swap the top and bottom ones and it steps up or down.
Picture: Drawn by Science Playground, after jagarikin · Wikipedia(opens in a new tab)