Many teachers visit this site looking for optical illusions to put in their lesson plans. This is a perfect example of an optical illusion that can be drawn. It’s another shape in the optical illusions collection that will twist and turn in your mind. The three dots will appear to be inside the object if you scan it from left to right, but then they’ll appear to be on the outside of this impossible shape if you scan it with your eyes from the right to left!
Sometimes, you’ll see a similar picture with the “devil’s pitchfork” showing that the trident cannot really insert into the three holes, because the trident is shown in a similar way.