This lesson illustrates how rivers such as the Tigris, Euphrates, Nile and Indus rivers increased the flow of resources, goods, services and ideas in the great civilizations from 4000 to 1000 B.C.E. In this lesson, students assume the role of producers in a three-round production simulation. They must produce (draw with markers or crayons) three distinct items—a hoe, a staff and a spear—in order to feed themselves and their families. In Round One, they are selfsufficient and must complete all of their work without access to other markets. In Round Two, they divide the responsibilities of production based on specialized skills. Trade is now permitted, but geographic barriers to trade obstruct it. In Round Three, the barriers are removed to represent the greater connectivity of markets made possible by rivers. Students experience how the improved flow results in surplus production and specialization, which brought many benefits to all those involved in voluntary trade.