Interestingly, one of the simplest optical illusion has been built in various sizes allowing to see it out of a book or your computers screen. Real life sculptures, I mean. Francis Tabary made most of these.
Don’t fall for this swimming pool, don’t fall into this swimming pool.
This is an art installation where you may think that there is a swimming pool, but only the surface of the water is present as a glass roof on top of an empty swimming pool.
Source: 21st Century Museum of Art of Kanazawa by Leandro Erlich.
Look at the following picture and count the blocks that are drawn there. Will you find more than 2 of them?