An economic side-effect. Externalities are costs or benefits arising from an economic activity that affect somebody other than the people engaged in the economic activity and are not reflected fully in prices. For instance, smoke pumped out by a factory may impose clean-up costs on nearby residents; bees kept to produce honey may pollinate plants belonging to a nearby farmer, thus boosting his crop. Because these costs and benefits do not form part of the calculations of the people deciding whether to go ahead with the economic activity they are a form of market failure, since the amount of the activity carried out if left to the free market will be an inefficient use of resources. If the externality is beneficial, the market will provide too little; if it is a cost, the market will supply too much. One potential solution is regulation: a ban, say. Another, when the externality is negative, is a tax on the activity or, if the externality is positive, a subsidy. But the most efficient solution to externalities is to require them to be included in the costings of those engaged in the economic activity, so there is self-regulation. For instance, the externality of pollution can be solved by creating property rights over clean air, entitling their owner to a fee if they are infringed by a factory pumping out smoke. According to the Coase theorem (named after a Nobel prize-winning economist, Ronald Coase), it does not matter who has ownership, so long as property rights are fully allocated and completely free trade of all property rights is possible.