I'm such a sucker for stuff like this. It is a bracelet that I picked up at the Chicago Blogher conference that I attended in 2009 on behalf of ZESPRI Green and Gold.
This simple piece of leather with some plastic beads has some pigment that is embedded in the plastic and when used in combination with the sun's energy, or rather its ultraviolet light it changes colour. Now these beads won't change (ie lightbulbs should not affect them) unless there is UV rays (the same ones that can burn your skin and are a source of radiation).
So now I'm keen to hang onto this piece and see how quickly it changes when I'm outside i.e. on a cloudy day, a sunny one, a rainy one and so forth. And what else is cool is that I can put them under by sunglasses, expose them to sunlight and if they stay plain white then my lens are giving me protection - and if not, then the UV rays are making their way into my eyeballs.
Plus I can even slather on some sunscreen onto the beads - the more they change colour, the less protection I get and vice versa - no change in colour means lots of protection.
How cool is that? So anyhow you can check out their website at www.stevespanglerscience.com.