If, in the “Rolling Circles” illusion, the circles appear to be spinning, you don't need to update your eyeglass prescription. Instead, take a look at this Web site on optical illusions from psychologists at the Autonomous University of Barcelona in Spain. Here you can follow step-by-step explanations of some common physical, physiological, and cognitive trompe l'oeil. The site allows visitors to manipulate the illusions until their secrets are transparent. You can battle perspective to predict line lengths in Ponzo's illusion or play with the brightness in the spinning wheels trick. The circles appear to revolve because their luminance differs from that of the background.