Evidence is basically everything that is used to determine or demonstrate the truth of what is asserted in a case. Evidence is collected by the police or investigators and is documented and numbered. During a pre-trial period called discovery, the parties exchange evidence and begin to create a strategy for trial. During trial, evidence is used to prove either material facts of the case or elements of a crime. Many people think evidence is only physical items, but oral testimony is also evidence. Witnesses may testify to what they saw or discuss their knowledge of a particular situation that is important to the outcome of the case. Not all evidence is allowed into trial. Each state has rules of evidence that will outline what is and is not admissible. Once evidence is admitted into court the jury or the judge makes a judgment call as to the credibility or reliability of evidence to formulate their opinion of guilt or liability.