One last thing about keyword optimization. Some words get ignored by the crawlers in their analysis of a page. These words, which are called stop words in SEO parlance, refer to common articles and prepositions such as the, a, or an.
There's another class of words also referred to as stop words. These are words that cause AdSense to stop displaying paying ads and to start displaying public service ads (PSAs) instead. There is no official list of these stop words, but words related to controversial, taboo, or sensitive subjects such as death and abortion are suspected of being stop words.