When a monopoly occurs because it is more efficient for one firm to serve an entire market than for two or more firms to do so, because of the sort of economies of scale available in that market. A common example is water distribution, in which the main cost is laying a network of pipes to deliver water. One firm can do the job at a lower average cost per customer than two firms with competing networks of pipes. Monopolies can arise unnaturally by a firm acquiring sole ownership of a resource that is essential to the production of a good or service, or by a government granting a firm the legal right to be the sole producer. Other unnatural monopolies occur when a firm is much more efficient than its rivals for reasons other than economies of scale. Unlike some other sorts of monopoly, natural monopolies have little chance of being driven out of a market by more efficient new entrants. Thus regulation of natural monopolies may be needed to protect their captive consumers.