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Cranberry Raft Engineering Challenge

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Something about the gorgeous deep red color of cranberries makes me really happy. Maybe it is because we most often enjoy them during special holiday meals. But they also make me happy for another reason. The availability of cranberries at the supermarket during the winter holidays gives a unique opportunity for a fun kid’s engineering design challenge!

Did you know that cranberries float? That is why bogs are flooded when it is cranberry harvest time. The air-filled berries float to the top and are easy to gather.

Cranberry Raft Engineering Challenge

Now that you know that cranberries float, you can challenge your kids to build a cranberry raft! In this open-ended engineering design challenge, they will build a raft using cranberries and test to see how much weight the raft can hold without sinking.

Materials for Cranberry Rafts:


Set Your Parameters

This is an open-ended challenge, meaning you can set as many, or as few, parameters as you choose. You can allow the use of as many materials as desired. Or, you can put limits on the materials, for example: “Using 10 toothpicks and 15 cranberries, build a raft that holds at least 10 quarters without sinking.”

It is also fun to allow a free build first, and then begin adding challenges later. This gives kids a chance to explore and understand the materials and their limits before designing a raft to specifications. A professional engineer would understand the materials before going into the design process or would test them through the design process, just like this.

Build, Design, and Test

The most fun comes from building and testing the rafts. What shape works the best? How will you keep your load from falling in the water? etc.

Once the rafts are tested, allow for some time to tweak and improve designs and try again. Kids love working on problems like this! You might want to engage in some friendly competition with them, or you could allow siblings (or friends, or students who participate in your co-op, etc.) to have a fun contest.

Engineering with Food

What other foods can you design an engineering challenge around? The weather isn’t always pleasant for heading outside in the winter and there are all sorts of specialty food items available during the time from Halloween through New Year.

Here are some ideas:

  • If you’re looking for more scientific fun with cranberries, visit this post for more cranberry science!
  • Or use gumdrops and toothpicks so see how high a structure you make.
  • How much weight can a chain of candy canes hold?
  • Which holds more weight, ribbon candy on its side, or upright?
  • Can you build a sturdy gingerbread house without using anything to hold it together (like frosting)?

The possibilities are almost endless!

If you’re looking for a great hands-on STEM investigation, you won’t want to miss this one about pop-up turkey timers and melting point. My upper-elementary and middle school students always enjoyed it in those days before winter vacation.


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