Not sure how well they work, but the idea is that they are clear until you shine light through them/on them at which stage they "glow" purple. They remind me of Transitions lenses for eyeglasses where the lenses are permeated with silver chloride that reacts with sunlight to go a dark colour - I'm guessing these work in a similar fashion. What concerned me with them was that one report on them I read (edit: and the product description as well now) said that they only work with actual sunlight, not incandescent light and I'm not certain that I want my TF toys in a display that requires sunlight to be effective (thinking specifically of say G1 Jazz and Red alert).