SUN cream is a fickle friend. It protects against burn-inducing ultraviolet (UV) light, but only for a period. And the first most people know of when that period is up is the sickening sensation that their skin is starting to catch fire—by which time it is too late to act. But Justin Gooding of the University of New South Wales, in Sydney, Australia, now thinks he has a solution for those who might otherwise risk overdoing it on Bondi Beach: a stick-on UV sensor that can be tuned to give warning when a new slap of protection would be advisable.