Relative color / value

If I do this right, a picture puzzle will appear. It is a good illustration of how color appears to change depending on the colors it is near. Assemble this puzzle, but don't place one of the blue pieces that goes into one of the right-hand corners. Before you finish, hold the piece and slide it over the other blue areas. It  should look like the moving piece is changing color, because the background is changing. Interesting, I think.