1) States have powers and their own governments yet come together under one federal power to form the United States of America.
2) Checks and balances- the executive checks the legislative, the legislative the judicial, etc. For example, the president can veto Congressional legislation, and the Supreme Court can declare laws and presidential actions unconstitutional. In the same way, Congress can impeach a president.
3) Amendments! There are currently 27. The first 10 are known as the bill of rights.
