83 Optical Illusions & Visual Phenomena can be a somewhat technical website. It explains why your eye does the things it does following many of the images. A lot of its illusions are interactive, allowing you to remove the part that makes it an illusion and see that, for example, the lines really are straight. In some of the more difficult to see images, it talks you through how to see it and tells you what to look for.
No wonder it’s so hard to make paintings. The brown tile in the center of the top face and the yellow tile in the center of the side facing left are the same color.
This is one of many illusions from Beau Lotto’s LottoLab Illusion pages. They say that their “AIM is to explain and explore HOW and WHY we see what we do.”