Giving money away for free is not behavior one expects from ordinary, rational Americans. But it’s something they do every day in massive numbers—that is if you consider the penny to be money. At store counters around the country, people will leave pennies for the next customer, something they’d never do with a dime or quarter or any piece of currency they actually value. The poor, pathetic penny has become clutter in the nation’s pockets, the irritating detritus of cash transactions that inconveniently don’t end in a 5 or 0. Pennies sit in jars around the country, waiting in desuetude...