Learn about crime scene investigation with three hands-on activities: look for clues at your own ‘crime scene,’ uncover fingerprints with dusting and cyanoacrylate fuming, and discover the colors of ink through chromatography.
Study a Crime Scene of Your Own
See how many clues you can identify in your own ‘crime scene.’ Choose a room (e.g., kitchen, living room, bedroom) or part of a room and go over it carefully, finding any trace evidence such as hair, clothing fibers, and chips of paint. You can collect these with a pair of tweezers and place them in envelopes or Ziplock bags to identify later. Are there any prints or scuff marks on the floor from shoes? Bits of soil or rock that might have been tracked in? To be thorough, record all of these clues and make sketches in an investigation notebook.
If you have a microscope, compare different kinds of hair at high power magnification. (You can also use a 10x or stronger magnifying glass.) Examine different cloth fibers, too – try cotton, wool, and rayon or acetate. Make a wet mount of the hair or fibers by putting a drop of water on a microscope slide, adding the specimen, and pressing a cover slip down on top. What does each specimen look like? Is it smooth or rough? How do the ends look? Compare miscellaneous hair and fibers you pick up from the carpet or couch. Can you tell what kinds of fibers they are? Where did they likely come from?
Check out any dental evidence in your crime scene. Then, if you have some willing suspects, make impressions of their bites and compare the impressions to the evidence you found. A simple way to make impressions is to carefully bite down into an apple or other soft food, but you can also bite into a folded sheet of white paper with a piece of carbon paper inside. Be sure to get both back and front teeth in the bite impression.
Prints All Over the Place
Collecting fingerprints is not that hard to do at home, and we don’t mean with the ink and stamp method! Some fingerprints are visible – you can see marks left on a surface by dirty or oily fingers. Dusting is usually used for this type. Other prints are latent – you can’t see them, but there are marks left by sweat and other organic residue from fingers. Fuming is often used for these.
If you have a magnifying glass, inspect your fingers. The unique patterns on your fingertips are caused by ridges in the dermis, the bottom layer of your skin. These patterns are fully developed in human beings just seven months after conception, while the fetus is still in the womb. The three typical patterns are loops, whorls, and arches. (Look at examples of different fingerprint patterns.) Your fingerprints are different than anyone else’s, but did you know that fingerprint patterns tend to run in the family? If your fingerprints are a whorled pattern, one of your parents probably has a whorled pattern, too. It’s just not exactly like yours!
To dust for fingerprints, sprinkle talcum powder or cornstarch on dark surfaces and cocoa powder on light surfaces (like the outside of a drinking glass) where there are visible prints. You can use a small paint or makeup brush with very soft bristles to gently swipe off the excess powder and leave the print. Use clear tape, sticky side down, to lift the print and then stick it to an opposite-colored paper. What kinds of patterns do you see?
Another method for collecting fingerprints is called fuming. Certain chemical fumes react with the sweat and other organic residue left in latent fingerprints. You can experiment with this yourself: all you need is an aluminum pie plate or square of aluminum foil folded in fourths, a glass jar, superglue, and a smooth object like a pen or a marker lid. (Take care with the superglue, and make sure you have an adult’s permission!) Wipe down the object, then hold it for a minute so that your fingers leave latent prints. Set the object inside the jar. Next, put several drops of superglue on the middle of the pie plate and turn the jar upside down over it. The strong chemical fumes from the cyanoacrylate in the glue will react with the residue from your fingers. You should see white fingerprint images on the object after a half hour or so. Professionals also use ninhydrin (which reacts with amino acids in latent prints) and silver nitrate powder developed under a UV light.
Fingerprint everyone in your house. What patterns are most common? Based on your latent and visible fingerprint collection, which surfaces ‘reveal’ prints best? (To make these fingerprint records, it will be easiest to use ink or marker rubbed onto the fingertips and then stamped onto a white paper or card.)
Who Wrote It?
Chromatography is used to identify different inks. Say someone committed a crime by changing the dollar amount on a check. Using chromatography, an investigator could tell whether more than one ink pen was used to write on the check and whether the suspect’s ink pen could have been used. How does it work? Well, ink is not really made up of one color: there are actually different pigments making up one ink. In chromatography, the ink is soaked in a solution so that the different pigments will ‘bleed’ apart and the true colors be revealed. (As you might guess, there is a drawback: the evidence is destroyed in the process.)
You can see how chromatography works by doing this experiment. Fill a tall glass half way with water. Cut 3-4 strips of filter paper or of a heavy paper towel and attach the ends to a stiff piece of wire or a stick that can rest over the top of the glass. Next, make a large dot of ink about 1/2 an inch from the bottom of the strips, using a different brand of black marker, felt-tip pen, or ink pen for each strip. Set the strips in the glass so that the ends touch the water but the ink dots are above the water level. As the water soaks up into the paper, the ink will begin to separate into different colors. Note that some inks are not water-soluble; if the ink does not bleed, try using either nail polish remover or rubbing alcohol (stronger solvents that can dissolve the bonds in the ink) instead of water.
You can also look at ‘suspect’ paper itself – are there watermarks or imprints from writing on top? Professionals also study handwriting and can analyze a sample of disguised writing to see if it has characteristics that match a suspect’s normal writing.