Illusionary Fun

I can't get enough of these kinds of things! The last one especially reminds me of the work of my favorite artist, M.C. Escher.

My dad used to show me stuff like this when I was little kid, usually from textbooks or journal articles that he was reading - or writing. The pictures fascinated me even then, though I couldn't understand a single word of the articles themselves (and still mostly can't). I remember one of my favorites showed dozens of different kinds of chairs; regular dining room chairs, stools, recliners, beanbag chairs, etc. There was no single visual factor that all of them had, other than being objects whose primary purpose was for people to sit on them. He told me that the average five-year-old child, when asked what the pictures had in common, could respond correctly that they were all chairs. Yet the most sophisticated computer in the world could not. I wonder how much the field has advanced since then?