Locard's Principle (sometimes called Locard's Principle of Exchange) is a fundamental part of modern forensics, and states simply that "every contact leaves a trace."

This is specifically used in relation to crime scenes. Effectively it means that anybody entering a crime scene will take something of that crime scene with them, as well as leaving something of them behind.

This can constitute something as obvious as a footprint, or a compression in a carpet, or can go right down to the molecular level.

These remnants can then be used to track the perpetrator of the crime. Often, a psychological profile of the criminal can be constructed from the traces he has left at a scene.

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