Optical illusions demonstrate how our brains can perceive things differently than how they truly are. The document discusses various optical illusions that play tricks on our vision, including symbols that appear to be one thing but are another, determining who looks taller in an image, counting the number of people in a picture, effects of visual fatigue, and how artworks by artists like Salvador Dali, Utagawa Kuniyoshi, and Arcimboldo make use of optical illusions. The document also briefly mentions gestalt laws of perception and the relationship between figures and backgrounds to perception.