An amendment to the Constitution is a change that can add to the Constitution or change an older part of it. An amendment can even overturn a previous amendment, as the 21st did to the 18th. There are a few methods to amend the Constitution, but the most common is to pass an amendment through the Congress, on a two-thirds vote. After that, the amendment goes to the states, and if three-quarters of the states pass the amendment, it is considered a part of the Constitution — it has been ratified.