The U.S. Constitution
The Constitution is the foundation of the United States government. It tells how the government is structured and how it works. The Constitution defines the powers of the different parts of the government, creating a system of checks and balances and division of power.
Read the Constitution, and learn more it here:
http://www.usconstitution.net/index.html
Learn more about checks and balances here:
http://bensguide.gpo.gov/3-5/government/branches.html
http://www.uscourts.gov/outreach/resources/separationofpowers.html
http://www.socialstudieshelp.com/Lesson_13_Notes.htm
http://www.usconstitution.net/consttop_cnb.html
http://www.congressforkids.net/Constitution_checksandbalances.htm