Locard's exchange principle

In forensic science, Locard's principle holds that the perpetrator of a crime will bring something into the crime scene and leave with something from it, and that both can be used as forensic evidence. Edmond Locard (1877–1966) was a pioneer in forensic science who became known as the Sherlock Holmes of Lyon, France.

Source: Wikipedia — Locard's exchange principle (CC BY-SA 4.0)