Actually, if you will follow the line with your finger, you will find that each is a complete circle. There is no spiral!
This is another fun picture. Squares A and B are the same shade of gray.
I know that you may have a more diffucult time believing me on this one. Right click on the image and save it to your computer. Then open the image with Paint or other similar program. Then just cut out a piece of one and move it to the other square. Or you can print the image and then cut or fold the paper to prove it to yourself. The contrast of light and dark created by the shadow fools our brains into believing that B is lighter than A.
And then there are those that can draw what is an impossible reality.Have a great day...but don't always trust your eyes!