Article 5 - Amendment
The Founding Fathers recognized that because of changing circumstances it might be necessary to change the Constitution. They wanted to make sure that this process would not happen too easily because that might make the government structure unstable. So they created a fairly complicated process for amending the Constitution, requiring the consent of 2/3 of both Houses of Congress and 3/4 of the state legislatures.
This seems to have worked out well. There are currently (as of August 1, 2006) 27 amendments to the Constitution. Ten of them (the Bill of Rights) were added in one swoop in 1791. Usually the amendments approved were for significant changes. However, when the 18th Amendment, an attempt to outlaw alcoholic beverages, was adopted, it was ignored so much that the 21st Amendment was passed to repeal it. Franklin Roosevelt's first Presidential race had as one of its main platform planks the repeal of the 18th Amendment. Even though our country was in the worst economic crisis in its history, the repeal of Prohibition was considered a huge issue.