HOW GOOD ARE YOUR DETECTIVE SKILLS?

For the budding detectives in your house, the following activities will introduce you to the study of forensic science.  The challenge will be to figure out “Who Stole the Chocolate?” 

One family member will secretly be the culprit and leave behind tell-tale clues, such as fingerprints, a message written in invisible ink, and a bitten piece of chocolate.  As other members of the family try to solve the case, they’ll be making decisions using forensic techniques they have learned, just like real detectives. These activities were taken from an excellent website called Who Dunnit: the case of the Barefoot Burgler, from cyberbee.com. 

Let’s get started ….

The crime scene set-up:  Think of how you will stage the crime scene to include a surface where a piece of thick chocolate candy, showing teeth marks, is left behind. Under or near the candy is a piece of paper that includes a secret message (you decide what the secret message is). There are sticky fingerprints on the paper. 

The devious plan:  Can you devise a reason for “stealing” the candy and leaving the note and fingerprints behind? Determine if one person will be the thief, OR if they have an accomplice, leading to two sets of prints and evidence.  Other family members can then try to solve the case, using forensic skills that identify the culprit.  As you can see, this activity can be as small or as large as you want it to be!  Feel free to simplify this activity to include just one or two of the skills.

The techniques used to solve the crime: Before studying the crime scene you must practice the skills needed in analyzing evidence.  Brief explanations are given below for three forensic skills.  After you have completed your analyses, see if you can answer the following questions, which will lead to solving the case.

What is written on the piece of paper?  (Secret Invisible Ink Messages)
Whose fingerprints are those?  (Fingerprinting)
Who took a bite of the candy?  (Teeth Impressions)

SKILL #1 - MAKING AN INVISIBLE INK MESSAGE

IMPORTANT: The culprit will leave a piece of paper at the crime scene, on which he or she has written a message in invisible ink.  The forensic team will use some chemistry to reveal the message.

Supplies needed are: paper, small paintbrushes or cotton swabs, baking soda, rubbing alcohol, and turmeric (a yellow spice).

To make the ink, thoroughly combine ½ cup water and 1 tablespoon baking soda.  Then write or draw a message on a piece of paper, using a small paintbrush or swab.  Let the paper fully dry (15 minutes or so should do it).

To reveal the hidden message, you will need to make a reagent (a substance used to cause a chemical reaction).  Thoroughly combine ½ cup rubbing alcohol with 1 teaspoon turmeric.  For this step it’s a good idea to put down a couple layers of newspaper so you don’t stain whatever work surface you’re using.  If you have messy kids, it may also be a good idea to give them aprons or something, because turmeric leaves a lovely bright yellow stain on everything it touches.  

Put the paper with the secret message on the newspaper.  Using a small paintbrush (the cotton swab is too small for this step), paint the sheet of paper with the reagent and see your message revealed in bright red ink.



Skill #2 - MAKING AND ANALYZING FINGERPRINTS       

IMPORTANT: The “thief” makes his or her fingerprints at the crime scene.  But, to solve the crime, all persons involved will have to have their fingerprints taken so that a comparison can be made.  

If you’ve ever wanted to collect clues like a real detective, you can get some practice dusting for fingerprints with this activity.  You will need: colored chalk, a butter knife, cornstarch, a small paint brush, tape, paper and a surface on which to leave your prints.  For this exercise, we suggest you leave fingerprints on the secret message paper, but feel free to change that to a different surface. 

Start by scraping your colored chalk with the butter knife to make a fine powder.  Even though butter knives aren’t very sharp, make sure an adult is there to help keep everybody safe.
Mix your powdered colored chalk with an equal portion of cornstarch.  This mixture will be your fingerprint powder.  

Fingerprints stick well on smooth, flat surfaces like mirrors or glass.  The cleaner the surface, the easier it will be to collect and see your fingerprints.  Test a variety of surfaces to see which ones work best.  You can try a table, a plastic cup, a metal pot, a kitchen counter, a cutting board … the possibilities are endless!  Or, as suggested, put fingerprints on the piece of paper with the invisible message.

After fingerprints have been made, this is how you will collect them. Use your brush to sprinkle some of your powder on the surfaces with the fingerprints.  Be careful not to touch the brush to the fingerprint itself; you don’t want to smear it.  Just hold the brush a little above the fingerprint and shake some powder on it.  You want to cover the whole print with powder.

After you’re done sprinkling, blow across the surface to remove any extra powder.  Too much powder on the surface will make a messy fingerprint that will be hard to see.

Stick a piece of clear tape on the powder left on the surface.  The oils from the skin on the culprit’s finger left a pattern on the surface.  The powder has now taken on the shape of that pattern and when the tape touches it, the powder will stick to the tape in the same pattern as the real fingerprint.

To best see the fingerprint, remove the tape and stick it on a piece of paper so you can see the chalk. You can experiment with different surfaces to see which ones show fingerprints best.  
Each person has unique fingerprints that are different from everyone else’s.  But many fingerprints have designs in common like swirls or loops.  You can learn more about the patterns of fingerprints at 

http://www.forensicsciencesimplified.org/prints/principles.html.



SKILL #3 – MAKING TEETH IMPRESSIONS

IMPORTANT:  The culprit will bite into a thick piece of chocolate and leave the chocolate piece, showing the teeth marks, at the crime scene.  But to solve the crime, all persons involved will have to make teeth impressions on a Styrofoam plate so that a comparison can be made.  
Forensic dentists assist in crime solving by studying teeth and teeth impressions. Dental records are often used to identify people. Because teeth are one of the hardest substances in the human body, they are frequently well preserved. Dental x-rays or records showing fillings, position of teeth, etc. can help forensic dentists find a match of teeth to the individual. Teeth impressions are used to identify unknown victims.

The procedures for making teeth impressions are:

1. Divide a Styrofoam plate into six equal wedges. Cut the wedges.
2. Take two of the wedges and stack them together. Cut off 1 inch from the pointed end of the wedges.
3. Place the two wedges into your mouth as far back as possible.
4. Bite down on the wedges firmly and then remove them.
5. Label the top and bottom wedges Top Teeth and Bottom Teeth.
6. Study the teeth impressions. Count the number of teeth in the top and bottom impressions.



What other characteristics of the impressions do you notice? Compare the top teeth impressions to the bottom. Are there teeth missing, spaces, chips, etc.?

Now, put all the evidence together … CONGRATULATIONS! You used forensics to solve the crime.  Forensic science is the study of objects that relate to a crime. The objects are “evidence”, and analyzing the evidence is what forensic scientists do. They observe, classify, compare, use numbers, measure, predict, interpret data, and draw inferences. Did you enjoy being a detective? Was this your “cup of tea”? Or rather, your “piece of chocolate”?

Learn more about the scientific practices we discussed at the links below:

Link 1 | Link 2 | Link 3