13. Another "false spiral" -- actually a series of concentric circles, hard as it may be to believe. To prove that there's no spiral, try tracing your finger around one of the circles. You'll see that it never moves inward or outward.

15. First, look at the image at a close distance, then look at it from 1 meter distance

17. Which direction is the cat rotating: clockwise or anticlockwise? Once again, different people will see different direction. I see it rotating in a clockwise direction. With practice, you can make it switch at will!