It's hard to be responsible when you're having fun in the sun. That's why a tireless piece of science, like this real-time UV exposure alarm, is a better companion at the beach than a great book.
Instructables user nmcclana uses an inverted light-emitting diode as a light sensor, based on the same technology that remote control makers use to determine when to turn on internal lights. It's a geeky electronic project, for sure, and requires buying some obscure parts and assembling them in pro-am fashion, but when it's built, "Mr Burns" is pretty darned cool. Set it to match your skin tone, put it in the same sun you're in, and, using World Health Organisation standards, the device will alert you when you're approaching the UV danger zone.
Know of another clever exposure monitor other than the hindsight of peeling skin? Tell us about it in the comments.
Build a Sunburn Alarm [Instructables]