SPF

I've always been uninformed about how the SPF numbers on sunscreen bottles work. I understood that it had something to do with exposure time, but I hadn't ever gotten the formula to work in the real sun.
I found this explanation online:

To figure out how long you can stay in the sun with a given SPF, use this equation:

Minutes to burn without sunscreen x SPF number = maximum sun exposure time

SPF numbers start at 2 and have just recently reached 70. With that number, a person who normally burns within 10 minutes can now enjoy 700 minutes -- almost 12 hours -- in the sun without any damage. Right?

? ...whatever.

SPF number means more sun-exposure time. It also indicates the level of UVB absorption, but this number doesn't increase exponentially, which can be confusing. For example, an SPF of 15 absorbs 93.3 percent of UVB rays, but an SPF of 30 absorbs 96.7 percent. The SPF number has doubled, but the absorption rate has increased by only 3.4 percent.
 
...the percentage of UVB absorbed and overall protection of the skin increases only slightly, but people may misinterpret these higher SPF numbers as a much higher level of protection or even a guarantee of all-day protection.

The bottom line is that a sunscreen with a higher SPF does offer higher protection against UVB rays, but once you get past SPF 30, protection doesn't increase dramatically, and the higher number may give you a false sense of protection. 

That explains some things. Also, after wondering why the dog in Victor's presentation had to have sunscreen applied 30 minutes early, I also discoverd from this website that it's true for humans too:

To be effective, sunscreen needs to be fully absorbed into the skin, so apply it 15 to 30 minutes before you even step into the sun.

There's more fascinating stuff about sunscreen on this website:

http://health.howstuffworks.com/skin-care/beauty/sun-care/spf1.htm