While this post has nothing to do with development, it is something that affects all of us. Well, at least those that pull them selves away from their computer or game console long enough to notice that there is a really large yellow star really close to our planet ;-)
After a long weekend of being out in direct sunlight, I found myself asking "Just how exactly does sunscreen work?". After a bit of searching, sunscreen works by chemically modifying your skin to reflect the UVB (the ultraviolet rays that cause sunburn) or convert the rays into another spectrum causing heat to be reflected. Also, I guess I would have known this if I'd read the instructions, but you should place sunscreen on at least 15 minutes prior to going out into the sun in order to allow the chemicals the necessary time to chemically react with your skin. If also found out what SPF actually means, it's not necessarily the protection, but rather a guage that says, using SPF 15 for example, that you can stay out 15x longer than without using protection. So if it takes you between 10-15 minutes to burn, you will be able to stay out between 2-3 hours when using sunscreen. Therefore, one of the reasons why it's recommended to reapply every 2-3 hours. ;-)
Here's a few (somewhat) helpful links discussing "How does sunscreen work?"