But it could save your life. The nine-millimeter-wide gadget, developed by Northwestern University and the cosmetics company L’Oréal, measures UVA and UVB rays that cause sunburns and skin cancer. When you sweep your smartphone over it, an app processes the data and shows you how much exposure you’ve gotten that day or accumulated over time. It made its debut at the Consumer Electronics Show this winter and is set to hit the market this spring. It will come in a range of colors and designs and is expected to cost $50 or less. Developers recommend sticking it on your thumbnail, but anywhere on your body that gets a lot of sun—and won’t look weird with a button-size tan line—will do.