A person who understands the scientific method can test to see if the facts of a scientific theory are correct. When statements have been tested and shown to be not false, they are said to be "true". The facts of each scientific theory have usually been questioned and tested for a very long time by many people. However, the critical point is that an agreed method has been used to show that the statement is not false. Because of this, we know that the knowledge of a scientific theory is fact and not opinion. Examples of scientific theories include the theory of relativity and the theory of evolution.