Give a Penny, Take a Penny

Give a Penny, Take a Penny By Clifford F. Thies from Mises.org
http://www.mises.org/story/2254

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At my supermarket, there's a machine that says "convert your coins into money," for which service it charges "only" 3 percent.
Aren't coins already money? And, why should it cost money to convert one form of money into another form?