Walton: Paper currency’s dirty little secret

Three months ago I noticed that I had only $7 in my wallet, a five and two ones. That seemed uncomfortably low, so I made a trip to my bank’s ATM machine to reload. I withdrew a hundred bucks from my savings, and suddenly my wallet was flush with cash. Here we are, three months later, and I still have $87 of that $100 left. Am I so frugal that in three months’ time I have only spent $13?