Cities have this trait as well: they are anchored systems of production and trade, allowing labor, production, commodity circulation, and consumption. They need to be situated on top of a resource base that allows them to function. This could be anything from good natural resources to favorable geography to educated populations. Cities also require a network of communication technologies for their operation. Commerce requires roads and shipping ports while police and fire protection require law enforcement and emergency response teams. All these needs can only be met by having a population with enough technical expertise to build or use such things. Last but not the least, cities need markets to function. This could be within their borders (internal) or between cities (external). Most goods need to be transported over long distances before they reach consumers' hands. This requires industries which produce more demand than supply (i.e., factories) as well as efficient transportation networks. In conclusion, cities are complex organisms that consist of different sectors (industry, commerce, services), regions (urban vs. rural), and groups (the wealthy vs. the poor). They require adequate resources to operate and maintain themselves. At the same time, cities need to be dynamic so they can attract economic development.