Organization occurs when people learn what can be accomplished by pooling their efforts, resources, power, knowledge, and identities. Adam Smith was the first to explain that when labor is divided in an organized way, groups outperform individuals. Their superior performance is due to greater efficiency that comes with the specialization of workers and coordination of their work tasks. But efficiency is not all that organization makes possible. Groups can accomplish things no individual acting alone could dream to do: space travel, for example. Of all the concepts used to explain, understand, and appreciate organization and its successes and failures, organizational structure has been around the longest and made the greatest impression on organization theory. Structure generically refers to the stable relationships among parts of a system or entity. For example, relationships between the foundation, frame, roof, and walls of a building give it the structure needed to stand upright and function as shelter, just as relationships between bones, organs, blood, and tissue structure a human body and enable its life-supporting functions of mobility, digestion, respiration, circulation, and so on.