Black Triangle Illusion
What to see and do:The picture below looks like a black triangle in front of three circles, right? Move the circles out of the way and you will see that there is no triangle there!
What's happening: Your eyes are seeing a common shape inside the three circles. Your brain assumes that there is a triangle blocking parts of three circles. However, there are just three cut out circles.
Credit: This illusion is based on an illusion at http://www.eyetricks.com