Imagine a friend gives you a free ticket to a concert on Saturday night. It seems as if the evening will cost you nothing. An economist would disagree. The ticket is free, but your Saturday night is not.

The reason is scarcity. Time, money, and energy are all limited, so choosing one thing always means giving up something else. Economists call the value of the best option you give up the opportunity cost of a choice. If you would otherwise have worked a five-hour shift and earned sixty dollars, the opportunity cost of the free concert is at least those sixty dollars. If you would have spent the night studying for a chemistry test, the cost might be a better grade.

Opportunity cost explains decisions that seem strange at first. Why might a skilled carpenter pay someone else to paint her house, even if she paints faster? Because every hour she spends painting is an hour she could have spent on carpentry jobs that pay more. Hiring a painter is not laziness; it is a way to avoid an expensive trade-off.

The idea also applies to big decisions, such as whether to go to college. The obvious costs are tuition, books, and fees. A less obvious cost is the wages a student does not earn while attending classes full-time. Economists include those lost wages because they are a real part of what the student gives up. Many studies have found that, on average, people with college degrees earn more over their lifetimes, which is why many people decide the trade is worth it. But looking at the opportunity cost helps a student compare options honestly, such as a two-year technical program, a four-year degree, or working while studying part-time.

None of this means people should calculate the dollar value of every hour. Some of the most important parts of a choice, like time with family or the satisfaction of learning, are hard to measure in money. The point is simply to ask a better question. Instead of asking "How much does this cost?" an economist asks "What am I giving up to get it?"

So go to the concert, if you want to. Just recognize that it was never completely free.