The true cost of something is what you give up to get it. This includes not only the money spent in buying (or doing) the something, but also the economic benefits (utility) that you did without because you bought (or did) that particular something and thus can no longer buy (or do) something else. For example, the opportunity cost of choosing to train as a lawyer is not merely the tuition fees, price of books, and so on, but also the fact that you are no longer able to spend your time holding down a salaried job or developing your skills as a footballer. These lost opportunities may represent a significant loss of utility. Going for a walk may appear to cost nothing, until you consider the opportunity forgone to use that time earning money. Everything you do has an opportunity cost (see shadow price). Economics is primarily about the efficient use of scarce resources, and the notion of opportunity cost plays a crucial part in ensuring that resources are indeed being used efficiently.