Second. the nsa does it, the post reports, to find unknown associates of people it's targeting for surveillance. >> let's say they're interested in tracking somebody here by the red dot. as that person moves around a path, a certain number of people are going to be following the same general direction. as they move, fewer and fewer people are going down the same exact path. and say fewer people are there and by the time they get to one other location, there's only one person who has been travelng the same path, the green dot right there. >> and they can track when two people arrive at the same...