Bill Gates writes on Gates Notes: The Blog of Bill Gates that when he entered Harvard in 1973, the drawing below "was basically how the global economy worked."
The big picture: "There are two assumptions you can make based on this chart. The first is still more or less true today: as demand for a product goes up, supply increases, and price goes down. If the price gets too high, demand falls. The sweet spot where the two lines intersect is called equilibrium. ... Everyone wins."