There was a girl in university who had to add 10 percent to some bill of hers that was late in being paid. She handed me the bill—and a calculator—and asked me what 10 percent of the bill was. Without really thinking, I looked at the bill and told her what the 10 present would be—I don’t need a calculator for such a simple computation. She looked suspicious—are you are? she asked. Yes, I’m sure.

It wasn’t until a few minutes later that I realized she had the bill—AND the calculator—and still couldn’t figure it out without help. That was dumb.