Squaring the Circle - A Perspective Demonstration
Look at it from one direction, it looks like a circle. From the opposite direction it looks like a square. (You have to be the right distance away from it to see it correctly.)
https://www.youtube.com/watch?v=oghD4rwloKA&feature=youtu.be
It just goes to show you that two people can look at something from different perspectives and see completely different things.
Inspired by a sculpture I saw. (see http://mocoloco.com/vote/squaring-the-circle-by-troika/ )
EDIT: My original version used a 13 degree construction angle. It had to be held rather close to see the circle and square. I have now uploaded a version with a 5 degree construction angle. It works at more comfortable distance.