How does it know?!?!

I have a gmail account that has a very limited use and I like the application very much for what it allows me to do. There is one thing I don't like, however. I think it's called adsense. From what I can tell, it uses keywords in correspondence to determine what kind of advertising will show up in the right margin next to the frame containing the e-mail. For example, if a novel was mentioned, an ad for a publisher or bookseller would pop up. But some of the ads are hard to link to anything that was mentioned in the correspondence and can be quite alarming, such as this one, which, apallingly, has come up way more than once (!):

10 Rules for Stomach Fat

Drop 9 lbs of Stomach Fat every 11 Days by Sticking with the 10 Rules.
www.FatLoss4Idiots.com/StomachFat