Tired eyes
Lilac chaser
Stare at the cross for 20 seconds. What happens?
A green dot seems to race around the circle, and then the pink dots vanish completely.
Why? The green is the afterimage of the pink, showing up in each gap. And when you don’t move your eyes, your brain stops seeing blurry things that don’t change.
Picture: TotoBaggins at English Wikipedia Later versions were…, CC BY-SA 3.0 · Wikipedia(opens in a new tab)