Consumption of a good by one person may raise the marginal costs other persons face in consuming the good which results in congestion. Whether a good is congested or not at any particular time depends upon its level of demand at that time (i.e., highway roads). Care must be given to distinguish between marginal social cost of consumption and the marginal cost of production. A purely nonrivalrous and noncongestible good exhibits zero marginal costs of consumption. Yet increments of the good (unless they occur naturally) require various factor inputs to produce.