Boomers like to whine about how “they ought to teach ‘x’ in high school.”

And it’s always something dumb. Like changing your oil. When I need to change my oil, I pay someone maybe $25 to do it. No mess, no fuss, and I can play Angry Birds while the guy finishes it. Need to do it myself? I will guarantee that there’s an easily found video to show me how.

But the one thing that always comes up in those lists is that kids need to learn how to balance a checkbook. Does anyone really still do this? Seriously? And again, if you really feel the need to do this, there’s how to videos and articles. It’s not that complicated. Really.

But economic stuff is really important, and while I hated almost every minute of high school and couldn’t wait to get out, there are things that everyone who’s 18 should know. Like sunk costs.

What’s a sunk cost? Glad you asked. A sunk cost is a cost that has already happened and for which you can’t get the cost back. The sunk cost fallacy is doing something dumb even though you know the cost is gone.

Example: you bought non-refundable tickets for a movie. 10 minutes into the film, you realize you’re bored out of your mind. The ticket price is a sunk cost. You cannot get it back. The smart action is to walk out. At that point, all you’ve lost is the cost of the tickets. Stay for the whole movie (because “I’ve already paid for it”) and now you’ve lost two things: the ticket price as well as two hours of your time.

People ignore this all the time. Someone buys a nonrefundable timeshare and three weeks later realizes it was a mistake. The smart move in this situation is to learn any lesson you can (“don’t go to timeshare presentations, no matter how much pizza they offer you”) and go on with your life. Spending your next 15 vacations at the Dreary Gulch Resort “because I’ve paid for it” is stupid. In this case, you’ve already lost the money. Don’t compound the loss by losing your time as well.

I’m not telling you that you shouldn’t try to get your money back when you’ve made a bad call. Asking for a refund, even when you’re initially told it’s nonrefundable, works sometimes even when you don’t think it will. At the very least, ask. But when you can’t, don’t go on and waste your time. Time is very, very valuable. And making right decisions about it is one of the best life choices you can make.