Optical illusions are the artist playing with your eyes and what you think you know. While they are often the most intriguing art they are also the most difficult to create. You need to be able to think of not only positive space, but what the shapes of the negative spaces look like and you need to figure out how to put the two together.
Here are a few examples that have been created by several different artists.
Do you have any ideas for a great optical illusion? What would that look like?