If you use non-chemically whitened paper and put MICR (magnetic) toner into the copier, you can VERY EASILY fool the change machines. Trust me on this one :)
Since 1990, US currency in denominations of $20 and up has had a small, metal-like strip buried in the left-hand side. If you hold the bill up to the light, and look for a verticle line running approx through the 'mint mark', you'll find it. Tearing the top of the bill around the line will let you remove this little bit of foil. The foil is lettered with the denomination of the bill. Collect 'em all! Of course, you don't put twenties in the change machine. Ob (weak) crypto reference: Has anyone else noticed that the NSA has plans to build a largish installation in Dallas in '95? Something like 6 *big* Cray machines. I have it on good word that E-Systems is involved. Jim