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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. 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 make them easy to gather. Visit this post for more fun science facts and lesson ideas about cranberries. 

Cranberry Raft Engineering Challenge

Now that you know that cranberries float, you can challenge your kids or students 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:

  • fresh cranberries
  • round, pointed toothpicks like these (flat toothpicks are not strong enough)
  • a basin or tub to put water in
  • uniform objects to test the raft (coins, metal washers, marbles, dice, etc.)


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, 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 their 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 work 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, especially if they can work with other kids or against them in some friendly competition.

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:

  • How high of a structure can you make with gumdrops and toothpicks?
  • How much weight can you hold with a chain of candy canes?
  • What 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.


My name is Sarah Benton Feitlinger and I am a science educator with over 10 years experience sharing science in nature and environmental centers, museums and schools. I have been studying science and nature in one way or another pretty much my whole life! I have a passion for making science understandable, and my goal is to make it accessible for everyone. My blog focuses on connecting current events in science to resources and activities for teachers, parents and students.


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