Cool Illusion

The squares marked A and B are the same shade of gray

Don’t believe it? here’s proof…
By joining the squares marked A and B with two vertical stripes of the same shade of gray, it becomes clear that both squares are the same.

You can test in Photoshop. Using the eyedropper tool you can determine that the RGB values of the grays in both square A and square B are 120-120-120.