Just before I retired from teaching, I had a class of “difficult” students, the kind that weren’t interested in anything I had to teach and didn’t want to listen to anything I had to say. I can’t remember where I got it, but I had a calendar with pictures that were called stereograms. I brought it in and had my students look at them, to see if they could figure out what made them tick. When they finally saw what was there, they were amazed. I’d forgotten all about them until they were mentioned in one of the Dick Francis books I’ve been reading. So I went on-line and sure enough, there they were. I still can’t quite figure out why they work, but work they do. And the more often one looks at the images, the easier it becomes to call them up. If you look at the center of the image and then sort of let the eyes glaze over, a bit like crossing the eyes, a three-dimensional image will appear. Amazing.
My search also took me to the more common optical illusions, such as the one below.