SPF is a number that stands for Sun Protective Factor. The highest is probably a 45 or sometimes a 60. The number tells you how long you can be out in the sun without burning. Obviously there are not 30 hours of sunlight in a day, so an SPF of 30 means that, if you would normally burn in 15 minutes without sunscreen, using an SPF 30 sunscreen would allow you to be out 30 times as long without burning - that is, if it stayed on perfectly. Unfortunately, sunscreen comes off. When the sunscreen comes off, you'll need to reapply it. I believe it's very important to use a sunscreen with at least a 30 SPF, especially in places like Florida or Texas. It's also very important to reapply it every two hours when you spend time outside.
