The next time you're buying something and a teenager is at the cash register (fast food, etc.), give them whole bills at first. Then when they punch in the amount you gave them to tally the change, give them the odd pennies, nickels, dimes to round off to the nearest dollar.
Be prepared to be disappointed.
On a side note, I did find one guy that was pretty intelligent. I started calling out numbers like 63 and 27/16th's and then he started throwing concrete rubble at me.....lol
When I was 9 I took a job as a paper boy (back when we had afternoon delivery and it was reasonably safe to do) and a little old lady would always make me count the change back to her. She always was a nice lady and I could tell that when I counted change back to other customers that they appreciated it.