According to Marx goods can only poorly fulfill the role of money. It may be added that according to the theoretical opinion of many economists representing the dominating theory, money is a medium which fulfills the role of a means of exchange. If that is so, which of the two, goods or money, are needed more in the world? As a matter of fact, besides being a means of exchange, money plays yet another role. In todays reality Marx' view of money as a means of exchange can only be understood as a fairy tale. But this fairy tale has to be realized in reality. It is actually the money itself which has the power to impose interest. Marx has not understood that the interest does not arise during the production process, and since he had not solved the question he got himself tangled up in contradictions. If the trade, which merchants are performing by their various purchases and sales, is equivalent, then, he believed, the secret must lie hidden in the purchases and sales of the merchants. But here is being shown in a practical example, that this is a mistake.