The waterfall illusion, scientifically known as the Motion Aftereffect (MAE), is a powerful visual phenomenon where stationary objects appear to move after you have spent time gazing at a moving stimulus. It was famously documented by Robert Addams in 1834 after he watched the Fall of Foyers in Scotland. When you stare at the downward rush of a waterfall for about 30 to 60 seconds and then quickly look at the static rocks beside it, the rocks will appear to drift upward. This happens because the neurons in your brain that are tuned to detect downward motion become fatigued or "habituated." When you look away, the "upward-detecting" neurons, which are still fresh, fire at a higher relative rate than the tired "downward" ones, creating the vivid sensation of movement in the opposite direction. It is one of the most studied examples of how our sensory systems adapt to the environment, proving that what we "see" is often a processed interpretation rather than a direct feed of reality.