Essentially, our brains — which would otherwise be overwhelmed — take shortcuts with the information they feed to our minds. Clever illusions reveal the shortcuts:
Illusions can be literal, physiological, or cognitive, depending on which aspect of your brain is the object of a con job on your vision.News, “Optical illusions: What causes them? Try some out!” at Mind Matters News (June 3, 2022)
Many more at the link.