Totally Tubular
Rolling up the bill into a tube like this is interesting enough on its own, but the tubes are also nice 'building' material for other designs.
(I once made the undercarraige of a Santa's sleigh using these little numbers, but that's another story.)
Start at one corner rolling as tightly as you can manage, at about the angle
shown. Continue rolling the entire bill, trying to maintain the tightest
possible roll while maintaining the same angle.
Once you've finished rolling the entire bill, as soon as you let go, the
bill 'relaxes' and unrolls a bit. The key to getting the tight roll shown
in the image below is to start rolling back the other way from the outside
corner. Don't unroll the first roll any more than necessary.
After rolling twice, the bill will remain in the tube shown above without tape or glue (or spit or ...) more or less indefinitely. (Old 'soggy' notes will not hold the roll.)