On a whim I stopped by the Bureau of Engraving and Printing. It's a good idea to get your ticket earlier in the day and come back at the scheduled time. They take you on a 30 minute tour through the building and show you how money is printed.
In case you are confused, like I was, there are 2 bureaus and 4 mints in the US. The bureaus print the paper money and the mints print coins.
Our guide told us that money really doesn't grow on trees because it is made of 25% linen and 75% cotton. Guides always have the cheesiest jokes, but that was interesting information. That explains why money doesn't get ruined in the washing machine.
On average, it costs 7.6 cents to make each note.
We weren't allowed to take pictures while walking through the building, but we could take pictures at the gift shop afterwards which had a few displays.
If you like money (and who doesn't) then this is a fun place to stop by for a short tour.