An optical illusion is a visually perceived image that is deceptive or misleading. This happens when the information gathered by the eye disagrees with how the brain processes that information.
How many black dots do you see?
What animal is this?
How many legs does the elephant have?
What do you see here?
Concentrate on the red dot. Are the circles rotating?
Are the holes inside or out?
Look at the flag for 15-20 seconds and then look at the white space below. Do you still see the flag?