I'm sure many of you already know the answer to this. Most of you will probably say "Sun Light" but there's more to it. I found this really interesting article on the subject and thought I would share it. Basically some chemists from the University of North Carolina At Chapel Hill did some studies on the subject. I would like to note that this story is quite old, as it is from 2001 but I'm sure it will enlighten a few people here. Other than that I hope you will enjoy!
http://www.sciencedaily.com/releases/2001/10/011018071707.htm
http://www.sciencedaily.com/releases/2001/10/011018071707.htm