Have you noticed that the little strip in the side of a $20 bill has started appearing in the lower denominations, namely $10 and $5. The latter is particularly useful. First, find a $5 bill that has the strip. Now use a pair of very small tweezers to pull this strip out. It is difficult, but with a little work, it will happen. Now use some clear (not foggy) scotch tape to tape this strip in approximatly the same location on the front side (the president side) of a $1 bill.
Go to a change machine that will still give you quaters, unless you really want tokens for the arcade...You'll get $5 in quarters.
Handy, but how's it really profitable? Get 4 $5 bills and 4 $1's. Pull the strips out off all of them and tape to the $1's. Change them, and then wrap the quarters in wrappers. Take them to the bank to get changed into $20, and then spend the $5's elsewhere (99% of all stores don't look for the strip in $5's and $10's).
See, the change machines made in the last 4-5 years use this strip to decide what it just ate. If it sees the strip, it's a $5, if no strip, then it's a $1. CAUTION: THIS IS A FEDERAL OFFENSE AS WELL AS A FELONY. THIS IS COUNTER- FITTING. I HIGHLY RECOMMEND AGAINST THIS.
Thanks to PeTro249000 for the info