who stole the cake

Who Stole the Cake? – Activity

Who could have done this heinous crime?!

After the meeting of the Play Outside Forever group, someone stole the special cake for the party tonight!  All there is left is a few crumbs on this plate.  There were 12 people at the group meeting and all of them are suspects.  We have interviewed them and gathered some evidence but we need you the Science Detectives to help us find the thief!  Gathering and testing evidence from a crime scene is called Forensic Science.

Forensic science is the broad field of study that involves applying science to criminal and civil laws, such as during a criminal investigation.  It involves physics, chemistry, biology, computer science and engineering.

We are using forensic science in this lesson by applying chemistry to a crime scene investigation.

So lets get this mystery solved!

Had so much fun you want to do it again?  Looking to do it in 2 groups?

Well you are in luck!!  We have built in 2 separate scenarios for this mystery.  It is the same story and process but tweak a few variables and you have a completely different answer.

These instructions are only for Scenario #1.  Scenario #2 is included in the PDF.  But you set them both us the same way so no worries.

Supplies

  • Plate with crumbs  – 1
  • Cornstarch  – 1 or 2 Tablespoons
  • Baking soda  – 1 or 2 Tablespoons
  • Cream of Tartar  – 1 or 2 Tablespoons
  • Vinegar  – about 1 Tablespoon
  • Iodine solution  – about 1 Tablespoon
  • Ice cube tray  – 1
  • Bowl or food storage container  – 1
  • Paper clips  – 3 or 4
  • Paper towel  – 1
  • 91% Isopropyl alcohol or rubbing alcohol  – 1/2+ cups
  • Suspect cards  – 1 each printed front & back
  • Thief fingerprint & note cards  – 1
  • Science Detective page  – 1 per student
  • 3 different brands of black marker
  • Timer (watch or phone would work)
  • Measuring tape (optional)

This experiment needs 3 different brands of black marker. It can be permanent or washable.

Scenarios #1 & #2 supplies

The scenarios take all the same supplies except:

  • Scrap of green fabric or string (scenario 1)
  • Scrap of black fabric or string (scenario 2)

Preparation – Scenario #1

This seems like a lot of prep but it’s not!  We tried to be as detailed as possible to create a good mystery.

Printing

The Crime Scene

  • Choose a spot for the initial crime scene.
  • Place some kind of crumbs on an empty plate to show evidence of the stolen cake.

Clue #1 – time

Clue #1 is an eyewitness statement about time.  The location needs to be exactly 1 minute from the crime scene.

  • Choose a spot away from the crime scene and practice walking to it and timing for 1 minute.

**Only the location of Clue #1 matters.  Put all of the other clues wherever you choose.  Just spread them out in your backyard or at the park so they kids get some walking in.

Clue #2 – cloth

  • Place a scrap of green fabric or string 4.5 feet (1.4 meters) off the ground. You can make it look like it was snagged on a fence or bush.

Clue #3 – note

markers

For the note,  you will need 3 different brands of black marker. They can be permanent, washable, or dry erase.

  • Cut a strip of paper towel about 1 inch (2.5 cm) wide and several inches long. It needs to be long enough to reach from the bottom of your bowl or food storage container to the top and be paperclipped to the edge.
  • Draw a wide band of color across the strip of paper towel about 1 inch (2.5 cm) from the bottom.
  • Label the top of the strip so you know it is a “mystery sample” from the marker that wrote the crime scene note.  I used the letter M.
  • For young kids, cut out 4 or 5 extra strips for their experiment.  This is a great step to let the kids do themselves if they can uses scissors.
  • Now you need to label each marker as Marker #1, #2, or #3.  This corresponds with the marker numbers on the suspect cards.  The marker you used to make the “mystery sample” MUST be labeled as Marker #1 or you’ll get the wrong suspect.

Clue #4 – powder

Temporarily mark an ice cube tray with numbers 1-12.
Add about 12 teaspoon of the indicated powder to the tray.

#1 – cream of tartar
#3 – backing soda
#5 – cornstarch
#7 – backing soda
#9 – cream of tartar
#11 – cream of tartar

#2 – cream of tartar
#4 – cream of tartar
#6 – cornstarch
#8 – cream of tartar
#10 – baking soda
#12 – cornstarch

Put about 1/2 teaspoon of cornstarch in a separate container as the “mystery sample” from the crime scene.

powders

Activity Time

Introduce the Scene of the Crime

This intro will happen at the crime scene with the crumb filled plate as evidence.

After the meeting of the Play Outside Forever group, someone stole the special cake for the party tonight!  All there is left is a few crumbs on this plate.  There were 12 people at the group meeting and all of them are suspects.  We have interviewed them and gathered some evidence but we need you the Science Detectives to help us find the thief!  Gathering and testing evidence from a crime scene is called Forensic Science.

Forensic science is the broad field of study that involves applying science to criminal and civil laws, such as during a criminal investigation.  It involves physics, chemistry, biology, computer science and engineering.

We are using forensic science in this lesson by applying chemistry to a crime scene investigation.  So lets get this mystery solved!

Now give out the Science Detective sheets where the detectives can write down their observations for each clue.

Clue #1 – time

Someone spotted the thief at 10:04am.  You need to walk to that spot you picked during the prep with the timer and decide how long it takes to get there.  No one was seen running so you have to walk.  This should take a minute or less.

Use what you know of the suspects time last seen at the crime scene to eliminate suspects who did not have enough time to walk to that spot.

  • What time did they leave the crime scene?
  • How much time between when the suspect left to when someone was spotted at 10:04am?
  • Who had time to walk that far before 10:04am?

Example:  Suspect #1 – Alice was last seen at the crime scene at 9:58am.  That leaves 6 minutes to get to the eyewitness location.  Plenty of time.  She is still a suspect.

investigating

Deductive Reasoning

The way we are eliminating suspects is called deductive reasoning.  Deductive reasoning uses a group of general known facts to draw a conclusion.

Example:

  • Alice left the crime scene at 9:58am (fact)
  • An eyewitness saw the thief at 10:04am (fact)
  • It take 1 minute to walk from the crime scene to Clue #1. (fact).

Using these facts, we can deduce that Alice had plenty of time to walk to Clue #1 and is still a suspect.

Clue #2 – cloth

Now head over to Clue #2.  Point out the cloth as the next clue.  If you have a tape measure, measure the exact height of the cloth from the ground.

  • How far is it off the ground?
  • Would this cloth come from a shirt or pants?
  • What color is it?
  • Which suspect could have left this color of fabric at this height?

Eliminate more suspects due to what you know about Clue #2.

Clue #3 – note

Now for Clue #3.  Show the note found at the crime scene (found on pg. 22).  We have a sample of the marker that was used to write the note.  We also have markers that were found at the homes of each suspect.  We are going to do an experiment to decide which of these markers was used to write the note.  We will then eliminate any suspects that do not have the right kind of marker.

You already prepared the Mystery marker sample.  Have the kids prepare their marker samples the same way.   The kids will make 1 sample strip for each marker (3 total) that they will compare to the Mystery marker sample.

  • Fill the bottom of your bowl or food storage container with about 1/8 inch (3 mm) of alcohol.
  • Paper clip the prepared sample strips to the side of the bowl with just the end of the sample touching the alcohol. DO NOT LET THE MARKER DIRECTLY TOUCH THE ALCOHOL. It will ruin the sample.  The alcohol needs to climb slowly up the paper towel.
chrom setup

Chromatography

Now do you remember our Color Climb chromatography lesson?  Well now we get to use all the things we learned there to help identify our thief!

chrom results

** You can move onto Clue #4 while this experiment runs.  It will take about 15+ minutes.  **

Once the experiment has set for about 10-15 minutes, compare the samples.

  • Are they the same or different?
  • Are the black markers really all just black?
  • Which marker sample matches the “mystery sample” from the crime scene note?

Eliminate more suspects based on your observations  about the markers.

Clue #4 – powder

The next clue is a mysterious white powder found at the crime scene.  White powders were also found at the homes of each of the 12 suspects.  Now we need to figure out if the powder from the scene is the same as the powder from the suspects just like in the Powder Mystery lesson.

First add a few drops of iodine to each sample.

  • Does the mystery sample react with the iodine?
  • Can you tell which other samples are the same as the mystery sample yet?

Next add vinegar to each sample.  It should still bubble and fizz even though we have already added the iodine.

  • Does the mystery sample react with the iodine?
  • Can you tell which other samples are the same as the mystery sample yet?

Eliminate suspects by comparing the powder sample with the mystery powder.

Clue #5 – fingerprints

We have just gotten a fingerprint!  Now that we are down to the last few suspects it will be easy to compare fingerprints  of each suspect.  Every person has a unique fingerprint so the Mystery Print will only match 1 suspect.

The students will compare the fingerprints of the suspects to Mystery Print 1.

  • What are the similarities and differences in the fingerprints?
  • Which fingerprint matches exactly?

You’ve caught the suspect!!

The Science Detectives have uncovered the real cake thief!

  • Which suspect matches your observations?

(see the answer on page 26 for scenario 1).

Being detective is hard work!  But science makes it a little easier.

solved

Grab the PDF!

For answers to our little cake mystery, suspect cards, investigator sheet, and Scenario #2, download the full PDF.

cake thief suspects

This activity is part of Who Stole the Cake? [Unit 2].  Jump over to the unit main page to see all lessons and activities.

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