Lines along the spokes’ sides: no spoke ever crosses them.
Tricks of motion
A wheel that turns without turning
Which way is the wheel turning?
It isn’t. It seems to turn clockwise, on and on, but every spoke stays where it is.
Why? Each spoke fades lighter or darker while its dark and light edges swap sides. Your eye’s motion sensors take each change for a little step, always the same way round.
Picture: Drawn by Science Playground, after jagarikin · Wikipedia(opens in a new tab)