Edmund Locard, commonly referred to as the father of forensic science, said that whenever two objects come into contact, there will be an exchange between the two.
Some of object #1 will be left on object #2, and vice versa. The harder the impact, the bigger the exchange. This is the main principle of forensic science, and particularly, forensic chemistry and toxicology.

Forensic chemistry, although similar, is slightly different than forensic toxicology, in that forensic chemists deal with non-biological substances, such as paint, glass, or liquids, and forensic toxicologists deal mainly with body fluids to determine, for example, the level of alcohol or drugs a person has consumed and how these toxins would affect the person.