Interesting in relation to the diffusion of new ideas. I tried two bills, listed below.
Nature 439, 462-465 (26 January 2006) | doi:10.1038/nature04292;
The scaling laws of human travel
D. Brockmann1,2, L. Hufnagel3 and T. Geisel1,2,4
The website wheresgeorge.com invites its users to enter the serial numbers of their US dollar bills and track them across America and beyond. Why? "For fun and because it had not been done yet", they say. But the dataset accumulated since December 1998 has provided the ideal raw material to test the mathematical laws underlying human travel, and that has important implications for the epidemiology of infectious diseases.