How Full is Choc-full? A delicious activity from Puzzles and Projects

March 22nd, 2022

Make yummy rocky road chocolate while learning about packing density maths in this delicious hands-on activity from Puzzles and Projects.
Two pieces of delicious rocky road artfully stacked on top of one another.

Make a tasty treat out of maths with this packing density activity!


Book cover.

Puzzles and Projects, by CSIRO’s Double Helix magazine team.

Hungering for a maths activity you can really sink your teeth into? We’ve got a treat for you in this edited extract from Puzzles and Projects.

You’ll need an adult to help with this activity, but the delicious result is worth it!


Is a dish full of marshmallows really full? This yummy rocky road recipe shows that there’s still heaps of space in a ‘full’ dish.

Maths can always help you find a little extra room for dessert!


Safety: Ask an adult to supervise this activity. Use clean hands and equipment. Take care using the microwave and when cutting with the sharp knife.


You will need:


  • Butter or spray oil
  • 2 cups marshmallows
  • 1 cup rice puffs
  • 400 g dark chocolate buttons


  • A small dish, about 2 cups in size
  • Baking paper
  • Metal spoon
  • Large clean towel
  • Metal spoon
  • Microwave-safe glass or ceramic bowl
  • Sharp knife
  • Chopping board


What to do:

Step 1. Grease the small dish with butter or spray oil.

Step 2. Line the dish with baking paper.

A square tupperware container lined with baking paper.


Step 3. Put the marshmallows into the dish, so they reach the top. Is the dish really full?

Tupperware container lined with baking paper and filled with large marshmallows.


Step 4. Pour the rice puffs into the dish. Shake the dish so they get into all the cracks. Is the dish really full?

Tupperware container filled with large marshmallows, with rice bubbles filling up the gaps.


Step 5. Place the dark chocolate buttons into a clean, dry microwave-safe glass or ceramic bowl.

Step 6. Use the microwave to heat the chocolate on medium for one minute.

Step 7. Carefully stir the chocolate with a clean, dry metal spoon.

Step 8. Repeat steps 6 and 7 until the chocolate is just melted and smooth. The bowl can get quite warm, so be careful when removing it from the microwave. Leave it to cool slightly if needed.

A bowl of melted chocolate with a stirring spoon.


Step 9. Pour the chocolate into the dish. Gently stir the contents to let the chocolate fill all the gaps. Is the dish full now?

The container of marshmallows and rice bubbles has been filled even further with melted chocolate, which has taken up all the air gaps.


Step 10. Put the dish into the fridge and let it set for an hour.

Step 11. Turn the rocky road out onto the chopping board. Ask an adult to help you cut it into pieces using the knife. Enjoy!

Slicing the cooled rocky road with a sharp knife on a cutting board.


What’s happening?

In this activity, you’ve packed quite a lot into a small dish. Sometimes mathematicians and scientists need to know how tightly packed things are. The number they use to measure this is known as the packing density.

Packing density measures the proportion of the container that’s actually filled with stuff, rather than gaps. Solid rock has a packing density of 1, because it has no gaps. Lightly packed rice puffs might have a packing density of ½, meaning 1 cup would contain ½ cup of rice puffs, and ½ cup of air.

One way to increase packing density is to use a mixture of different-sized objects. The smaller rice puffs help fill the gaps between the bigger marshmallows. Of course, in this recipe, there’s still plenty of space for the chocolate!

Big chopped chunks of delicious rocky road.

We wish all maths problems could be worked out with such a tasty solution! (Photo: Jasmine Fellows, CSIRO)


Puzzles and Projects, by CSIRO's Double Helix magazine team.

Written by the team behind CSIRO’s Double Helix magazine, Puzzles and Projects is packed full of fun (and sometimes delicious) hands-on experiments, brainteasers, quizzes and comics. It offers hours of entertainment, sparked by the wonders of science, technology, engineering and maths (STEM).

The Double Helix team has a long-standing reputation for delivering expertly written, fascinating and fun science material for young people, with an aim to foster an interest in STEM.

Puzzles and Projects is available from our website or all good bookstores.


Other books by the Double Helix team:

Book cover of Hands-on Science: 50 Kids' Activities from CSIRO.

Hands-on Science: 50 Kids’ Activities from CSIRO

Book cover of More Hands-on Science: 50 Amazing Kids' Activities from CSIRO.

More Hands-on Science: 50 Amazing Kids’ Activities from CSIRO

Double Helix Magazine:

A print copy of a Double Helix magazine issue, next to a tablet showing the digital issue.

Double Helix magazine is perfect for enquiring minds and young scientists aged 8–13 years.

Inside every issue you’ll find articles, activities, puzzles, comics and prizes galore.

Subscribe online today to receive each new issue, published every six weeks in print or digital.



Double Helix Extra:

And for even more fun activities, sign up to Double Helix Extra, our free fortnightly email newsletter for students, teachers, and anyone who loves STEM. Each email includes a quiz, a brainteaser, news and a hands-on activity.