A good example can be seen with Youtube. On a fresh computer with IP address, I was given an old game video from a friend. Finished watching it and talked other

A good example can be seen with Youtube. On a fresh computer with IP address, I was given an old game video from a friend. Finished watching it and talked other stuff. An hour later, when I went to youtube.com (main page) to look for a different old game video, I noticed the frontpage suggestions were the same as alongside when I watched the video. Quite a basic guess of what the user wants but seems effective to me. Although I have an account with youtube, I rarely log in. What's the point?