Fingerprints and solving a crime

I thought my son might like to solve a mystery, whilst learning a little about fingerprints and forensic science.  To start with, I explained a bit about fingerprints.  Everyone has unique fingerprints, i.e. they are different to everyone else. The patterns can look like a loop, a whorl or an arch, but the exact pattern of lines depends on the person.  We started with looking at his fingerprints, using a black washable ink pad and a magnifying glass. His are a loop, although it was pretty hard to see them as he tended to smudge the fingerprints when he picked his finger back up off the paper slightly sideways.  Still, when combined with looking at his fingertips which were still slightly inky and easier to see the patterns, I think this was enough to convince him that we've got patterns on our fingertips which I don't think he'd ever noticed or thought about before.

Looking at his fingerprints

Then we moved onto solving a crime!  The back story was that someone had stolen Teddy the bear, and he had to figure out who!  He donned his police dressing up outfit, and looked mildly concerned that only Teddy's jacket was left.  A water glass had moved at the same time as Teddy went missing, so maybe the thief had left some fingerprints on the glass?  I gave him a small amount of cocoa powder and a ball of cotton wool, and asked showed him how to lightly dust the glass with the cotton wool dipped in the cocoa.  I'd rubbed a bit of vegetable oil on my fingertips before I touched the glass (I'm giving away the answer to who the thief was here...) to make my fingerprints really easy to find, and the cocoa sticks to the prints, but not elsewhere on the glass, making them really visible. He seemed quite surprised to find the fingerprints were so visible.

Revealing fingerprints with cocoa powder

The next part of the activity was to lift the prints.  For this, we used sellotape.  If you cut a small piece and take care to hold it by the corners, you can stick it over the print on the glass then peel it off, and the cocoa powder retains the shape of the fingerprint.  If you then stick it on white paper, you can see the fingerprint clearly. When I tested this out myself, it worked perfectly, but when combined with slightly clumsy little hands the results weren't brilliant.  

Lifting fingerprints with sellotape

He loved cutting the sellotape - I recently bought him some scissors that actually cut properly and he now enjoys using scissors.  The sticking bit was a little harder and he wobbled the sellotape a bit on most of the prints which made them less well-defined than the ones I took.  Still, we talked about how the one print on its own must be the thumb and the four together were the fingers, with the smallest one showing which was the little finger.

Comparing to our fingerprints

The next bit was to identify the suspect.  He seemed really quite worried that his Teddy had gone (oops!), so I explained that the two suspects who had been identified were Mummy and Daddy.  This seemed quite a relief and he was happy to continue solving the mystery!  I'd put my own fingerprints on a sheet of paper, and persuaded my husband to do the same - one of us has a loop pattern and the other a whorl, although at a first glance they're not really easy to distinguish.  My son had a look at these with a magnifying glass, and whilst clearer than his own, they weren't easy to spot the patterns.  We compared them to the big drawings I'd done of the shapes, and then tried to compare the lifted prints to the ink ones.  It was beyond his observation skills really, and we resorted to comparing the size of the prints which pointed to me as the culprit.  I was then 'arrested', interrogated and revealed the location of the missing bear.



This didn't work perfectly, but it still provided an introduction to fingerprints, and how the differences between them can be used to work out who has touched something.  We'll come back to it when he's a bit older with slightly better fine motor skills, and I will try and line up a few more friends or family to provide prints to give more 'culprits' to examine along with a more convincing plot!

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