In his beautiful TEDx talk, John Sutherland, an officer in London's police department, explains a principle in forensic science called Locard's exchange principle. Developed by Dr. Edmond Locard, known as the Sherlock Holmes of France, this principle has a simple premise: every contact leaves a trace. In other words, every criminal leaves a trace behind him. One forensic expert put it this way:

Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or his footprints, but his hair, the fibers from his clothes, the glass he breaks … the paint he scratches, the blood … he deposits or collects … This is evidence that does not forget.

Sutherland explains how this principle applies not just to forensic science but to all human relationships:

Every time two people come into contact with one another an exchange takes place. Whether between lifelong friends or passing strangers, we encourage, we ignore, we hold out a hand, or we withdraw it. We walk towards or we walk away. We bless or we curse… And every single contact leaves a trace. The way that we treat and regard one another matters. It really matters.