Dirk Brockmann is a theoretical physicist from Germany studying population mobility in the United States. Brockmann noted that folks in the United States have a tendency to move around a lot and he was curious about where the heck Americans go and with whom they interact the most.
Brockmann's American friend, Dennis, suggested that he take a look a data from WheresGeorge.com. That's a site where participants log information on their dollar bills so that they can be tracked as they change hands.
So Brockmann used over 1 million reports from "Where's George?" to study the movement of 464,670 bills and came up with this map. The darker the blue line is, the more self-contained that part of the United States was with the bills tracked. In other words, money rarely traveled beyond those virtual lines. In the Seattle area, for example, only 7.8 percent of the bills logged in the site moved more than 500 miles away over a two week period.
But that isn't the only way to assess American population mobility. Take Xiaoji Chen's map called "The Connected States of America" picture in the gallery. It used data from AT&T's mobile network to show delineations that happen to be pretty similar to Brockmann's "Whom Do You Hang With" map.
Here's one of the many questions that these maps raise: Why don't Texas and Oklahoma interact more, either by phone or with their money? You can read more about Brockmann's study here.