Locard’s principle is concerned:
Ref. Textbook of forensic medicine. Krishnan Vij. Page. 302
 
Locard Principle:

When any two objects come into contact, there is always a transfer of material from each object on the other.
Some examples are fingerprints, footprints,hair, tool marks etc.
Theory of relativity
Theory of decomposition
Theory of trauma
Theory of exchagnge