[Lee Stewart]

The $5, $10, $20, and $50 bills have all been redesigned in the last decade or so, with the Federal Reserve adding color and watermarks to outsmart counterfeiters. But the dollar bill has remained unchanged since 1963. The reason why it's not been updated, according to the U.S. Treasury Department, is that this denomination "is infrequently counterfeited."

But another possible reason to take into account is likely the lobbying done by the vending-machine industry, which would have to redesign its machines to accommodate new bills should the current design get an overhaul.