They can even show ads that are related to the content on the web page! Are you on a page that is about breeds of dogs you might want to adopt? Why not buy a Halti collar, and a package of dog training sessions, and donate to the RSPCA?

This is what Google's advertising product started out as, basically automated magazine advertising at scale; it turned into this perverse tracking system once everyone was hooked onto free web content and nobody could get away from it.