One of my favorite things that was shown to me during my elementary age years was an optical illusion. It was the very famous Rubin Vase (pictured below) which you can see in either two ways - as a vase or as two faces that are facing each other.
Trees, nature's condos for the little guys, are not just a pretty picture. Squirrels, birds, and bugs count on them for a place to live. (I know the lens is about trees and not art. But I love that face-in-the-trees illusion artwork.