When it comes to money, Moms have a classic warning for their kids: "Don't put that in your mouth; you don't know where it's been."

Now the founders of WheresGeorge.com can actually tell you. They developed a way to track the movement of dollar bills across the country.

Here's how it works. On every U.S. dollar bill in circulation is a 10- or 11-digit serial code that is unique to that bill. When you go to Where's George, you enter the bill's demoniation ($1, $5, $10, etc.), bill series, the serial number, zip code and whether you're have possession of the bill at that moment.

According to the Charlotte Observer, Where's George will track how a bill moves across the country. You will probably get a more detailed history of a bill if you are lucky enough to come across one that has the "Track me at WheresGeorge.com" stamp on it.

I wanted to see how this works first-hand, so I pulled a one dollar bill out of my pocket and scrolled over to Where's George. I created an account and entered the information it asked about the dollar bill. Where's George created a unique URL to keep track of my dollar bill, which you can see here. Apparently, I am the first person to have registered this dollar bill, which isn't much fun to look at.

But Where's George has a registry of its most active dollar bills, which is a lot more interesting to see. The most active bill in circulation is this one, which has traveled nearly 5,000 miles to eight different states. But the last time anyone heard from this bill was in 2005, so it might be missing in action.

As for whether the bills have been in the back pocket of a pair of dirty jeans, or handled by someone who hasn't washed his hands, or between someone's mattress, Where's George can't tell you that. But I would still take mom's advice if I were you.

Photo: C. Sherburne/PhotoLink/Getty Images