Herring Illusion
Look at the long horizontal lines below. Are they straight or curved? Click the button to see. Try moving the spiral pattern around. Watch what happens to the lines! Now try moving the lines.
The long horizontal lines look curved, but they are really straight and parallel. The curving near the middle is caused by the background of sloping lines.
If you want to know more about the Herring Illusion, read the explanation.