The problem with opportunity cost is that opportunity cost is divided among many, many things.

What reminds you in your environment about saving? Nothing.

The things that motivate us are to help other people, to feel that we're useful, to feel that we're getting better, to feel that we are kind of living to our potential, to get a sense of meaning. All of those things are positive.

We're actually trying to develop an iPhone app, now that the Droid is out, we'll do it for that as well, if we ever learn how to program on this thing. But the idea is that to make money concrete. So, you can do this app, and it's not out there, but you can do the app. And you say, "I like vacation in the Bahamas, shoes, lattes, and books." And now, when you are tempted to buy something, that thing translates in terms of the things you are interested in.

Imagine you owe on five credit cards, you owe five debts. So which debt should you pay first? And the answer is very simple: You should pay the one with the highest interest rate first. But that's not what people do.

You can think about life as a battle between you and a doughnut shop. The doughnut shop wants you to eat another doughnut and pay the money, and you want to do it in the short term, but in the long term it's not good for you either financially or from a health perspective.

A very simple bad decision is to get into debt. And that is very expensive.