A Brownie Guide science evening: root nodules, carbon dioxide and chocolate

Outside of my “scientific life” I volunteer with Girlguiding as an assistant leader with a local Brownie Guide unit. Every week for about an hour and a half I help run a range of activities for a group of 7-10 year old girls.

Last week we had a science evening. At the start of the session we split the girls into three groups to rotate around three activities:

Root nodules

This activity was all about my research on Medicago truncatula and the symbiosis (or friendship) it forms with soil bacteria. The idea of this activity was to show them some of the nodules found on the plant roots (where the bacteria live and supply nutrients to the plant) and then give them an opportunity to dig up some plants and find some themselves.

Unfortunately, despite treating a batch of plants with bacteria several weeks earlier so that they should have produced nice big pink nodules, there were only a few nodules on the plants and they were still rather small and white. I had discovered this a day or so before the science evening but decided to go ahead regardless as it was a bit late to come up with a new plan. I found a plant from a previous experiment that had mature pink nodules on it to show the girls before I set them to the task of finding nodules in my “not so good batch” of plants.

Medicago truncatula root with nodules. Mature pink nodules highlighted in red circle. Immature white nodules in yellow. Image by ninjatacoshell By Ninjatacoshell (Own work) [CC-BY-SA-3.0] via Wikimedia Commons

Medicago truncatula root with nodules. Mature pink nodules highlighted in red circle. Immature nodules highlighted in yellow circle. Original image by Ninjatacoshell [CC-BY-SA-3.0] via Wikimedia Commons has been cropped and modified by S.Shailes and is released under the same licence.

They went ‘nodule hunting’ much enthusiasm and I was very impressed when many of them found some of the small white nodules (they were rather tricky to spot, even for me). We did have some cases of mistaken identity “is this a nodule?” “No, that is sand/a stone” but having never heard of nodules before that evening they did a very good job.

So alongside showing the girls what I study I also showed them that science doesn’t always go to plan! A good lesson…

Carbon Dioxide

Dry iceOne of the other leaders at my unit studied biochemistry at university and was very happy to run an activity all about carbon dioxide. I was busy with my nodule activity but from what I gather it involved some demonstrations to test whether water or carbon dioxide was better at putting out a shed fire (actually a lit candle the girls nicknamed Bob) and talking about what makes lemonade fizzy. And to finish off: some demonstrations involving dry ice (always a crowd pleaser!).


For the third activity the brownies went to the kitchen to be involved in “the World’s worst experiment”. It is an idea I got from a session I had helped to run with a group of undergraduates. The girls were told that they were doing an experiment to test whether eating chocolate makes you happy. The scientist running the experiment (aka a leader) started off by asking the girls to give a happiness score out of 10, recording the scores in a very haphazard fashion. Next, the girls were split into two groups and one group was given some chocolate (each girl was given a different amount and type of chocolate) to eat while the control (no chocolate) group were watching. Then the scientist went around again asking for happiness scores and then did some very dubious “calculations”.

After running the experiment the leader then discussed with the girls what they thought about the experiment. The girls picked up on a number of things that were bad about the experiment including:

  • ‘Chocolate’ group given different amounts of different type of chocolate.
  • ‘No chocolate’ group had to watch the other group eating the chocolate.
  • Just asking happiness score on scale 1-10 isn’t a very good way to measure happiness.

Of course there were a whole bunch of other things wrong with the experiment but I was really pleased that the girls spotted the ones they did. How many can you think of?

By coincidence this week Nature published an article called Twenty tips for interpreting scientific claims. Read the article or Alex B. Berezow’s blog post about it.

Note: At the end of the experiment we gave out chocolate to all the Brownies who hadn’t had any during it so no child went without!


6 thoughts on “A Brownie Guide science evening: root nodules, carbon dioxide and chocolate

  1. That’s a really nice idea! Did the girls understand what the root nodules are for?

    The chocolate experiment is great too! I’ll definitely bear it in mind next time I do some public engagement!

    • Yeah the girls understood that the bacteria help the plants get nutrients they need to grow and they live in the nodules. Mentioning that they are ‘good’ bacteria helped as they have heard of those in yogurts!

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