The moment when eyes cannot see

This is one of my favorite illusions. It tells us that seeing is mostly in our brain. Try this out, it is really interesting: Just look at the image below. What do you see?
Two gray rectangles, one is darker than other.. So what?
Now cover the border between the two regions by holding your pencil vertically over the boundary. What do you see now? One gray rectangle!!
Two regions magically combined to one single color. Our brain can tell the difference only when there are boundaries. If there is no border, it assumes simple all the same. So sometimes, we see only what our brain lets us to see…
(For details see Paul Hewitt’s Conceptual Physics book)