Here's a simple yet surprising optical illusion that can be made within minutes. It involves four different shades of blue that border each other. The shades go from lightest to darkest when viewed from left to right.
The illusion comes into play when an object is placed along the borders of each successive shade. Something strange occurs with our perception of the obvious difference in shades. In fact, neighboring shades appear to be precisely the same! Here's a video that showcases the full effect of the illusion: