Economists have two roles. As scientists they develop and test theories to explain the world around them. As policy advisers they use their theories to help change the world for the better. The focus of the preceding two chapters has been scientific. We have seen how supply and demand determine the price of a good and the quantity of the good sold. We have also seen how various events supply and demand and thereby change the equilibrium price and quantity. This chapter offers our first look at policy. Here we analyze various types of government policy using only the tools of supply and demand. As you will see the analysis yields some surprising insights. Policies often have effects that their architects did not intend or anticipate. We begin by considering policies that directly control prices. For example rent-control laws dictate maximum rent that landlords may charge tenants. Minimum-wage laws dictate the lowest wage that firms may pay work...