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Notes for the Instructor

A crime scene can tell the story of what happened when a crime was committed. The clues left at a scene can become evidence that someone has been there, even if – when investigators arrive – that person is no longer there.

Locard’s Exchange Principle suggests that the contact between two objects (such as a shoe and carpet) creates an opportunity for exchange. For example, when a shoe touches carpet, the shoe can deposit grit, dirt, or other material from its treads. At the same time, the shoe will pick up fibers from the carpet.

Dr. Edmond Locard was the director of the world’s first forensic laboratory in Lyon, France during the early 1900s. Dr. Locard was inspired by the writings of Sir Arthur Conan Doyle, the author of the Sherlock Holmes mysteries. He established several important ideas that are still a part of forensic studies today.

In this activity, students will learn about Locard’s Exchange Principle as they examine fibers found by Mrs. Randall at the crime scene. The investigation of fibers and hair is a very large field within forensic science.

There are many ways to study fibers found at a scene. Chromatography could be used to study the dyes used to color fibers. Sometimes fibers are burned to see what gases are emitted. Additionally, fibers can be studied under different types of light to reveal properties that might not be visible to the naked eye.

Fibers are considered a form of trace evidence, or small pieces of material that can link a suspect to the crime scene (here’s where Locard’s Exchange Principle comes into play). They can be transferred from the clothing of a suspect to the clothing of a victim or remain at the crime scene. Fibers can also transfer from carpets, beds or furniture at a crime scene onto the criminal or the victim.

Sometimes fibers are transferred directly from one source to another. This is called a primary transfer. An example of a primary transfer is when a person sits on a couch and fibers from the couch stick to the person’s shirt.

Fibers can also pass between more than one object. For example, dog hair might adhere to a person’s bag. When that person enters a school, the bag might come into contact with a rug and leave some of the dog hairs behind. This is called a secondary transfer. The dog hair went from the dog, to the bag, to the school rug.

In these scenarios the object that is the source of the shed is called the donor garment. The object that the shed ends up on is called the recipient garment. Some objects are better donor garments than others. Similarly, some objects are better recipient garments than others.

Fibers are most likely to be found immediately after contact. The more time that passes since contact, the less likely the chance to find transferred fibers. This is because the fibers can continue to be lost and transferred through movement and contact with additional objects. Because of this, investigators try to collect any evidence from a crime scene or the clothing of suspects as soon as possible.

In this lesson, students will be examining and comparing fibers obtained from the four suspects and from the cookie jar crime scene. This type of evidence is class evidence. It will not positively convict a suspect but will provide additional information that might be presented at a trial.

The activities in this lesson address Next Generation Science Standards practices of Planning and Carrying Out Investigations and Analyzing and Interpreting Data. In addition, they address Common Core Learning Standards. See the appendix on page 105 for more details.