A lot of new online programs like Facebook and Picasa use facial recognition technology to 'tag' pictures of people. The technology works on a simple principle. It picks out faces and groups ones that match each other, then you name that person and the tag is applied to all of the photos that person appears in later. The recognition technology on my Picasa that picks faces out of the photo is very sensitive. I'll be looking through all of the people I've photographed and see a young Tom Selleck's face. I'll wonder when I traveled back in time and shot this photo of him hanging out with my friends, then when I zoom out and look at the whole photo, it turns out that Magnum PI was on television in the background, and that Magnum's face was zoomed in on and indexed by my computer software.