It's Locard's theory.
One of basic principles of modern forensic science is something known as Locard's Theory. Simply stated, the theory is that every time something comes into contact with another thing, it either takes or leaves something of itself. Crime scene investigations are largely based around trying to determine what persons may have been present at the scene of the crime and what they may have come into contact with. Additionally, the scene will hold clues about what took place and how. Shoe prints, fingerprints, DNA, fibres, handwriting samples, blood splatter, tool marks, projectile impact patterns, and trajectory measurements all have a story to tell.