On the top of the cube on the left there are four squares that appear to be blue. On the cube on the right there are seven squares that appear to be yellow. If you look very carefully you will see that the blue squares on the left and the yellow squares on the right are actually gray, the exact same shade of gray. If you still don’t believe me you may have to cover up portions of the picture to compare them without the influence of the surrounding colors. I found it helpful to use a photo editing program to cut out the squares and compare them.
This illusion was created by Beau Lotto and demonstrates how we don’t perceive the world as it is, but we interpret what we see based on the context in which we see it. We have confidence in what we perceive because it works not because it is true.