I’ve always loved optical illusions. Anything, really, that reminds me that I am not my brain.
I recently came across a old, great illusion that I have loved and have always wished that I could tweak to find out what point I start to lose grasp of the circles. I built a visualization in JavaScript (original static image) using canvas today that allows you to alter the properties of the circles, sizing, and rotation and it allows you to do just that.
The key to the illusion is the alternating rotation of the squares in each circle guiding your eyes in conflicting directions making you visualize separate spirals. If you uncheck ‘alternateRotation’ you get a more subtle illusion of one attached spiral, which is still cool.