When prices or quantities change, economists generally seek an explanation in changes in the demand or supply curves. That approach assumes that the market was in equilibrium before the price or quantity change, and that the price adjusts quickly when the supply or demand move. Often, that assumption is a good starting point. The diagram below shows how movements in demand change the price and quantity if supply holds still. Higher demand makes both the price and the quantity sold go up.
Lower demand makes both the price and the quantity fall.
Now it's time for some visualization. Suppose instead that supply moves and demand stays still. The applet below activates when both of the above applets have been answered correctly. Version 4 or higher of IE or Netscape needed to view this applet, because it is stored in a .jar file for faster loading. For example, you might set a goal that everyone should get immunized, or that every pregnant woman should get prenatal care.