Take a look at these lines. The pairs seem to be moving somewhat independently. Now click the mouse or hit the space bar to show four "occluding" squares. With the occluders, it is now obvious that the lines are connected. Yet even knowing that, if we toggle the occluders back to the background color, it is difficult to perceive the lines as connected. This illusion comes from the collection from Michael Bach (http://www.michaelbach.de/ot/).
