After watching scary British crime dramas, my wife and I often turn to a rerun of The Big Bang Theory for comic relief. I find myself laughing almost as hard the 10th time I see an episode as I did the first time.

Last night, though, one comment of Penny’s stood out and it bugs me every time I hear it. She learns that even though she is the high earner in her marriage, her husband Leonard has squirreled away over $6,000 in a secret bank account. She’s upset.

I get that she’s upset. Trust and openness in a marriage are important. But here’s what I don’t get, or, rather, I get and disapprove of. She castigates Leonard because he’s not using the money for anything.

There are two possible meanings of “not using the money for anything.” The charitable interpretation is that he’s not investing it in something like the Vanguard Total Market Index. Even though the characters in Big Bang are getting old, they’re still not quite 40, so my advice would be to go all stock index.

But who thinks that’s what “I love these new shoes” Penny is thinking. The odds are over 90% that she meant Leonard is not spending the money whereas he or she could spend it.

Why does it bug me? Because I occasionally run into smart people who don’t understand the basics of saving and compound interest. They don’t get that by saving 10 to 15% of their gross income every year for 30 years or so, they can be, by their standards, fabulously wealthy.

In 1999, economists Dwight Lee and Richard McKenzie published Getting Rich in America: 8 Simple Rules for Building a Fortune and a Satisfying Life. In my review in the Wall Street Journal I called the book a “how to guide for becoming the millionaire next door.” The authors give 7 rules for becoming rich in America. (Their 8th rule is about balance.) Rule #3 is “Resist temptation.” Penny doesn’t do that. That upsets me to some degree. But what really gets me is her moral outrage at someone who does resist temptation and plans for old age.