What is Locard's Exchange Principle?

• A. The first law of thermodynamics
• B. Evidence degrades over time
• C. DNA is unique
• D. Every contact leaves a trace

Answer: (D)

Locard's Exchange Principle states that when two surfaces come into contact, material is exchanged between them. This idea is the basis for trace evidence: investigators look for tiny amounts of material transferred from a person to a scene or from a scene to a person, such as fibers, skin cells, or dust. The principle explains why you can link a suspect to a location or item because a transfer occurs whenever contact happens.

This is why the correct statement is that every contact leaves a trace. The other options reference different, unrelated ideas: the first law of thermodynamics is about energy conservation, not forensic transfer of material; evidence degrading over time is a general problem in handling evidence but not the principle of transfer; DNA being unique describes individuality, not the interaction of touching surfaces.