Where’s George? Interesting idea, not sure if it will work, but I tried it. You go to this site, enter the serial number of one or more bills of any denomination in your pocket, and write the URL of the site on the bill. If anyone who subsequently receives the bill notices the URL, logs on and enters their location, you’re notified by email and can track the meanderings of the currency. They report that they’re tracking over $2,000,000 in currency entered by over 175,000 registrants.