The basic neurobiology behind the 12-dot illusion
The way the eye interfaces with the brain never ceases to amaze me.  Another fun fact is that it even has it's own error correction mechanisms, one of which you can intentionally miscalibrate in order to see colors that aren't there for days, weeks, and sometimes even months after doing the calibration (which is acheived by looking at a very specific image pattern for a long time).  I would not recommend actually doing this, as those who have have reported back that the illusory colors become quite distracting and actually cause a bit of emotional distress after many months, but it's crazy that the brain has this kind of chromatic abboration error correction programmed in in the first place.  https://en.wikipedia.org/wiki/McCollough_effect