Optical illusions can be puzzling, amusing and headache-inducing (often all at the same time). Theories on how they work have been expounded on since the time of the Ancient Greeks, with philosopher Aristotle noticing that when you look at a waterfall and then shifted your gaze to static rocks nearby, it appeared as though the rocks were moving in the opposite direction than the waterfall. Since then, many people have tried to pin down exactly how they work. Is it the eye or the brain that is being tricked?