Peacocks, perhaps nature's most trippy bird, shake their tail feathers when it's time to attract a new mate. Why? Shaking those feathers -- called "train-rattling" -- causes an illusion where the eye-like circles on the feathers become more prominent, seemingly floating outward and hanging in the air. Those circles exist to lure in peahens, but have fascinated more than a few humans, too. Now a new study has taken a closer look at this 'train-rattling' dance and uncovered a few equally fascinating secrets about how it works.