Remember that whole "where is George?" site? You know, the one where you enter the serial number of a dollar bill and find out where it's been? Turns out it's actually useful for something:

Dirk Brockmann and colleagues used an online project called www.wheresgeorge.com (George Washington's image is on the $1 bill) to track the movements of dollar bills by serial number. Visitors to the site enter the serial number of banknotes in their possession and can see where else the note may have been.

The team tracked 464,670 dollar bills across the US using 1,033,095 individual reports. The fact the notes are carried by people suggests it is a good way of modelling other things that people may carry, including disease.

Mine are very easy to track, describing the shortest possible path from my wallet to Ellen's purse. Unless she wants to give me some lunch money, that is.