It is because solving a crime is never easy. It involves a very complex process of evidence gathering, scientific analysis of the evidence and logical deduction. Therefore, the single most important key to crime solving is evidence. Evidences are the trails and markings left behind by a criminal in the conduct of the crime, for example, the fingerprint, the footprint, the saliva. And so, by scientifically analyzing these trails, we are able to come to a conclusion as to how the crime was committed and who committed the crime.