Monthly Archives: January 2016

“How Do We Know That?”


I’m teaching this 12 week geometry class focusing on 3-dimensional figures. It’s a brand new class, like many at Baxter Academy, so I get to make it up as I go. Since our focus is on 3-dimensional figures, I thought I would begin with some Platonic solids. So I found some nets of the solids that my students could cut and fold. Once they had them constructed, there was a lot of recognition of the different shapes and, even though I was calling them tetrahedron, octahedron, and so on, many of my students began referring to them as if they were dice: D4, D8, D12, D20. Anyway, I must have made some statement about there only being 5 Platonic solids, and they now had the complete set. One student asked, “How do we know that? How do we know that there are only 5?” Great question, right?

I really felt that before we could go down the road of answering that question, my students needed a bit more knowledge and exploration around these shapes, and maybe some thinking around tiling the plane would help, too. So we spent some time trying to draw them, counting faces, edges, and vertices, visualizing what they might look like with vertices cut off, unfolding them into nets, and wondering why regular hexagons tiled a plane, but regular pentagons did not. We played around with the sides – a lot – and even talked about this thing called vertex angle defect. Then we returned to the question of why only five. Students were able to connect the need for some defect (angles totaling less than 360 degrees) and the ability to create a 3-dimensional figure. Through the investigation, they were able to see that the only combinations of regular polygons that worked (by sharing a vertex) would be 3, 4, and 5 equilateral triangles, 3 squares, and 3 regular pentagons. They could give solid reasons why 6 triangles, 4 squares, 4 pentagons, and any number of other regular polygons could not be used to create a new Platonic solid.

I had not anticipated this question, and had not included it in my plans. But, because it was asked, thankfully, by a student, it pushed us into thinking more deeply about these shapes (and their definition). And, ultimately, my students were able to answer the “why only five” question for themselves.


Filed under MTBoS Challenge, problem solving, teaching

Stacking Pennies


There are lots of things I love about my classes – my students are at the top, but I also am enjoying teaching a course for the 2nd or 3rd time. This activity is for a statistics course called “Designing Experiments and Studies.” It’s a course that lasts for one trimester (about 12 weeks) and we have just completed week 4. The Penny Stacking activity is an introduction to experiments. I ran this lesson last week (Jan 12 & 13).

The rules of penny stacking are simple:

  • You can only touch one penny at a time.
  • Once the penny is placed on the stack, you cannot move it.
  • Stack as many pennies as you can without having the stack fall over.
  • Each student stacks pennies once, with either their dominant hand or their non-dominant hand.
  • Students are randomly selected to stack pennies with either their dominant or non-dominant hand.

Before embarking on the experiment, we made the following predictions:

  • More pennies would be stacked with the dominant hand (although a few students disagreed and thought the results would be the same for both hands).
  • A few students thought the ratio of pennies stacked with dominant hand to non-dominant hand would be 3 to 1.
  • The range of pennies stacked with dominant hand would be 15-45 pennies, while the penny stacks from the non-dominant hand would range from 10-35 pennies.

Then it’s off to conduct the experiment. This is pretty tricky given that up to four students sit at a table and our building is so old that if someone walks across the floor upstairs, our floor will shake. Here are the results:

penny stacksHow would you interpret these results?

We calculated the means: dominant -> 30.9 pennies; non-dominant -> 26.25 pennies. It’s clear that, on average, the number of pennies stacked with the dominant hand is greater than the number of pennies stacked with the non-dominant hand. But is that difference in the means (of 4.65 pennies) significant? Is it unusually large? Is it more than what we might expect from randomizing the results?

To check this out, we randomize the results. Each pair of students received a stack of cards. Each card had a result. The partners shuffled up the cards and dealt them out in a stack of 10 (for the dominant hand) and a stack of 8 (for the non-dominant hand), calculated the means, and then subtracted (dominant – non-dominant). They each did this a couple of times and we made a histogram from the results of the randomization test.


(It’s a Google Sheets histogram – I don’t know how to get rid of the space between the bars)

If you look at our difference of 4.65 compared to these randomized results, it looks pretty common – not at all unusual – to get such a result. If you think that our randomization test was too small (with only 24 randomizations), then you can use the Randomization Distribution tool from Core Math Tools, a free suite of tools available from NCTM. And it’s the only tool that I know that runs this test effectively. Here are the results from 1000 runs, just like the card shuffling but faster.

more pennies

penny summaryAnd you can even get summary statistics that show that our result was within 1 standard deviation of the mean of the results that we got from randomizing the data. Not a very unusual result at all.

We followed this up on Thursday with an experiment inspired by an example from NCTM’s Focus in High School Mathematics: Reasoning and Sense Making – memorizing three letter “words.” Based on the experiment described in the book, I created random lists of three letter words and three letter “words.” The lists of words were meaningful, like cat, dog, act, tap, while the lists of “words” were nonsense, like nbg, rji, pxe, ghl. Students were randomly assigned to receive either a list of meaningful words or a list of nonsense words. They were then given 60 seconds to memorize as many words as possible. Like with the penny stacking, I made them predict what they thought the results might be. What would your predictions be?


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Filed under MTBoS Challenge, teaching

Working Together


Like all schools, my school has committees. And, like all schools, my school has a student government. But, unlike other schools, our student government is a single body: the Student Senate, comprised of one representative from each advisory. This year, the Student Senate split into subcommittees to align themselves with the faculty standing committees. I’m on the Academics Committee – the group charged with looking at curriculum & standards and how they are aligned. On Monday, we met with the Senate subcommittee during lunch to discuss different ways that students can demonstrate what they’ve learned. We talked about the possibility of students creating portfolios and presenting their portfolios to teachers in a particular learning area to have them assess which standards the student has met, and to what level. So, while the teachers keep working on tightening up our standards & curriculum (we’re halfway through year three), our students are developing a portfolio review process.


Filed under Baxter, MTBoS Challenge

About Me

It would appear that I can’t resist a challenge from the #MTBoS. And so, mtbos blogthis January, I’ve been challenged to update my “About Me” page. Feel free to check it out. I’ve updated it with a bit about the school where I teach and what happens when I talk about teaching at a public charter high school.

By the way, we will be graduating our first class this year, and that’s pretty exciting. Even more exciting is the fact that they’re actually getting accepted to college! (Of course they are. But as the first group, this is still pretty exciting for us.)

I’m hoping to blog a bit more this year. A bit more than once a month (or less). But I’m not promising anything. I’m still building brand new classes – which takes a ton of time. I have a couple of drafts still in the pipeline that need some attention. Maybe I’ll get to them soon.

In the meantime. Thanks for the push #MTBoS (and Tina C). And Happy New Year!

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