I recently added Google text ads on this site more out of curiosity than greed. The ads are not placed randomly. Software analyzes the content of the page and chooses appropriate ads that should in theory be on the top of mind for the readers. It has been fun to see which ads appear on this site.
A recent post of mine was titled Football Coaches are Running the World. Soon after, ads started to appear for sport psychology and coaching manuals. A little while ago I had written about matters scientific and immediately started to see ads for laboratory and optical equipment. A friend of mine told me that she had clicked on a couple of those ads. For days and days after it seemed that there were nothing but optical ads. It seems that clicked ads reflect a successful placement so more ads like it are immediately funneled in.
I did a search for a book written by an old professor of mine called Hegel's Ladder. I found a book review page and was very much amused by the ads on that page. Six out of the eight ads were for ladder hardware products. Software just ain't that smart yet.