A good that no consumer can be excluded from using if it is supplied, and for which consumption by one consumer does not reduce the quantity available for consumption by any other. A public good is therefore non-excludable and non-rivalrous. As a consequence of these properties public goods cause market failure. In practice, it is difficult to find a pure public good, i.e. one that satisfies the two conditions exactly. Public goods that are excludable, but at a cost, or suffer from congestion when too many consumers try to use them simultaneously, are called impure public goods; examples are parks and roads. Public goods are not necessarily desirable; undesirable ones are sometimes called public bads, e.g. polluted air. See also club; Lindahl equilibrium; preference revelation; Samuelson rule.