Mind Blown

[Cross Posted on the One Good Thing Teach blog]

Setup: We’ve been talking about perpendicular bisectors in various contexts in geometry. But they were just making observations and working on some simple proofs.

Last night in Geometry, students were tasked with the following:


It turns out that #3 is impossible, and #4 is possible with some guess and check. This sets up the background to have kids see something neat.

Then they are asked:



And now they see that for a triangle, the perpendicular bisectors of the sides all meet at a point. And that is rare and weird.  They then were asked to look at the point that the perpendicular bisectors meet at and the vertices of the triangles and make a conjecture.


Only one student “saw” it. It was fascinating for me that it was so hard for everyone else to see it! Others had conjectures that might have been true for right triangles or isosceles triangles or equilateral triangles… but not that were universally true.

For the rest of the class, to get them there, I did the following:


This was a huge setup for my “one good thing.” There were gasps, and one student said, and I quite, “MIND BLOWN.”

This weekend they are going to try to figure out what the what is up?!

PS. Yes, I am fairly certain that the setup of having students see the rarity of perpendicular bisectors meeting at a point, as well as having them look and fail to see something inside a set of seemingly random points was crucial for the big reveal. In fact, the fact that they didn’t discover it on it’s own was so powerful when they ended up seeing it.

UPDATE: The file I used is here.

You Spin Me Right Round: A Challenge for Geometry Teachers

We are now delving into some interesting coordinate geometry. We’re also beginning to use patty paper. And today the other geometry teacher and I had an awesomely fun conversation that revoles around something you might find it fun to think about.

Here’s the impetus/setup.

We have just finished talking about translations, and we’re moving on to reflections and rotations. When we introduce rotations to kids, we give them a backwards problem pretty early on: here is a figure and here is the rotation of the figure. Try to find the center of rotation. Use patty paper and guess and check.



This is an awesome exercise (inmyhumbleopinion) because it has kids use patty paper, it has them kinesthetically see the rotation, and it gives them immediate feedback on whether the point they thought was the center of rotation truly is the center of rotation. Simple, sweet, forces some thought.

This is our intro. We have a few more exercises in our problem packet similar to this. But then we move on. Le sigh. This felt wholly unsatisfying to us, because at its heart we left our kids hanging. We never get at the obvious question. How do I find the center of rotation without guessing and checking.

How do we take this introduction and make it mathematics?

The Challenge

[Baby challenge: Figure out how to find the center of rotation, given any figure and a rotated figure. This is not totally simple, okay? I’m calling it a baby challenge only because the next challenge is sooooo hard!]

Imagine you’re a geometry teacher, and you want students to discover a “foolproof method” to coming up with the center of rotation (given an original figure and a rotated figure). You also want students to be able to understand (deeply) and articulate why this method gives you the center of rotation.

What do you do to achieve these twin goals?

(This is what the other teacher and I were discussing today, and having loads of fun doing it!)

image (17)

You don’t need to answer this question in the comments. (Though you’re welcome to throw down any ideas.) I’m actually not looking for advice. (We’re well on our way to coming up with an answer to this.) I just thought it would be a fun thought exercise for you, if you like thinking of lesson planning/curriculum design. This is the type of stuff I love thinking about — when I have time!

This is backwards planning at it’s most fun, in my opinion. I have a deep result that is abstract and hard to grasp. I have very concrete 9th graders who I want to get from knowing almost nothing to discovering, understanding, and marveling at this great mathematical insight. How do I get from Point A to Point B?



Attacks and Counterattacks in Geometry

It’s been a long while since I’ve posted. It isn’t because I have nothing to post about! I’ve just been sooooo busy. This is the first year I’m teaching Geometry, and I’m working with the other teacher to turn it on it’s head. Completely. We haven’t cracked the textbook yet.

We started off the year with a very conceptual beginning, focusing on the importance of words, definitions, and classification. As you might have remembered from our first day activity, we have also been sprinkling in a good amount of conjecturing. [1]

I want to share one activity that I thought was not only was engaging, but led to really interesting discussions in my classroom.


Part I: Defining & Counter-attacking

On the second day of class, I had each of my geometry groups try to come up with a definition for the following words:


This is actually really challenging. I mean: you yourself, try to define a triangle without looking it up, or even more challenging, a polygon. Before starting, groups were told that other groups would try to find fault with their definitions, so they should be as specific and clear as possible.

Some things different groups wrote (all are problematic for various reasons):

Circle: “A circle is a closed figure where all the points are an equal distance from the epicenter, and starts and ends at the same point.”

Triangle: “A triangle is a 3-sided, 3-angled shape with straight lines that connect to the endpoints. Also, all the angles add to 180 degrees.”

Polygon: “Any shape that has exclusively straight edges that are all connected, and has to have at least three angles.”

Each group then passed their three definitions to a different group. And that new group was tasked with finding a counter-attack to these definitions. What this means is they needed to draw something that satisfies the definition they were given, but is not a circle (or triangle, or polygon). Those trying to counter-attack were allowed to read the definitions they were given in any way that seemed reasonable to them.

We then had a class discussion. Students publicly posted their group’s definitions (they were written on giant whiteboards), and then those with counter-attacks were allowed to present them to the whole class. When the counter-attacks were presented, it was interesting how the discussions unfolded. The original group often wanted to defend their definition, and state why the counter-attack was incorrect. Then a short spontaneous discussion would occur.

At the end of the discussion, I found myself often being arbiter and passing judgment on each counter-attack: “yes, this counter-attack works, because …” or “no, this counter-attack doesn’t work, because…” I felt the kids needed to know (a) whether the counter-attack really did satisfy all parts of the group’s definition, and (b) whether the counter-attack was using a fair reading of the group’s definition. When I said the counter-attack was valid, the group who found the counter-attack was elated! And when I said the counter-attack was invalid, the group who wrote the definition was elated! It became a bit of a spontaneous contest.

What was awesome was the subtleties they ended up talking about when trying to find the counter-attacks. When talking about the circles or polygons, for example, they realized that we have to say this is a 2D figure, otherwise there are many other curves that would work. When talking about triangles, saying the figure had three angles was problematic because there are 3 interior angles and 3 exterior angles. For triangles and polygons, students realized how crucial it was to say that the figures were closed. I was so impressed with how they were really trying to attend to precision in this task.


Part II: Understanding The Textbook Definitions

Eventually, we looked at a textbook’s definitions for these three words.




It took us a while to understand these definitions, and why the particular language was chosen. The polygon definition was especially challenging — especially the second half!


Part III: Taking Things Further

I started the next class with the following DO NOW:


Although I thought this would be easy for them, it was interesting to see that they found this challenging and abstract.

We also came up with the following two questions for a mini-quiz we gave:




The ending of our first unit involved students coming up with their own definitions for the bunch of quadrilaterals (kite, dart, square, trapezoid, rectangle, rhombus, convex quadrilateral, concave quadrilateral, isosceles trapezoid, parallelogram). This opening activity was designed to make that exercise easier when we got to there. Specifically, it was designed to show them that clear and precise language is important to communicate your ideas, and it isn’t easy to come up with clear and precise languageThings that we “think we know” are really quite hard to pin down… Like what a circle, triangle, and polygon are.


UPDATE: I found some examples of “counterattacks” that my kids drew for various definitions, so I figured I’d scan them in for posterity… We came up with more when we were having our discussion.

And here is a random picture I took in class with two of the whiteboards:


[1] I’m finding this to be a really rewarding thing to have sprinkled in. I’m learning it’s challenging for students to be able to try to make a potential conclusion from a number of examples. But in fact, isn’t that a crucial skill in mathematics? We see a number of examples of something, we decide on a very plausible conjecture, and then we try to reason out why that conjecture is true (or come to realize it isn’t true)?

Interesting conjectures

Today was my first day of classes — thirty minutes with each class just as a get to know you. Of course you know me. I dive right in and we did math in all of my classes.

The class I’m most nervous about this year is Geometry. It’s my first time teaching the subject, and it’s my first time teaching 9th graders. I had planned a paper folding activity which would get the students noticing and wondering. And importantly, then conjecturing.

Here are a quick snips from the paper folding activity.

The big idea was to get kids to realize that no matter which two points you chose initial to make that first fold, you’ll always end up with some things that are true in your final figure. Each student at a group folded two different pieces of paper — differently. Each group then looked at all their paper folds and made some great observations.

The couple that stood out to me:

When you make the folds, you will always end up with a pentagon. Some of the kids had a really small side, so they didn’t “see” the pentagon, but with a little prompting they did.

The five sided figure has three right angles. The two at the top (the corners of the original sheet of paper) and the point at the bottom. The point at the bottom is always going to be a right angle. 

As a class, people shared their group’s observations. And then we focused on that third right angle. THAT IS WEIRD.

And so I sent the kids off to try to come up with some reasoning for why that third angle is always a right angle.

And this is where I had my good moments. This is not an easy question for the very first day of class. And with the 8-10 minutes they had, no group completely made a perfectly sound set of reasoning. But so many were making statements that were getting them closer to the answer.

A few groups noticed that the two folded triangles looked like they were always similar. (Indeed, I had totally missed that when I first did this problem…) Some kids were able to come up with the logic that if the triangles were similar, then they could actually prove the bottom point was 90 degrees. Loved it! (As of yet, no kid or group has proved that the triangles are similar.

And a few groups were also noting that when you unfold the paper, there is something really remarkable about the bottom of the page, with the two creases emanating from that point (which is the vertex of that 90 degree angle when folded). They noted that somehow — by folding up the two triangles — they are splitting the bottom of the page (a 180 degree angle) into three angles, where the middle angle is always 90 degrees.So they have a 90 degree angle, and then two smaller angles that add up to 90 degrees. I said: that’s awesome. Now how do you know that middle angle is 90 degrees, no matter where the creases are? (As of yet, no kid or group has explained this halving.)

They are getting there.

Even though I was nervous about this being too hard and even though I wanted to provide more hints and more structure… I let things be. I wanted it to be tough. It wasn’t about the answer, but about the process of talking with each other and thinking and persevering and reasoning.

In fact, I wrote on the board that kids can ask me for one hint to help them if they got stuck. No group asked for a hint!

I am excited to see what happens when I next see them (Monday). I am having them try to figure this out at home, and write up whatever they could discover. If they could figure out the reasoning, great. If not, what did they think about and attempt.

My one good thing from today was watching my kids think aloud and struggle aloud and come up with really interesting ideas. Ideas I thought they might come up with, and ideas that were fresh to my eyes!

Here are all our paper foldings!


[Note: I am posting this both on the one-good-thing blog and my blog, because it both is a good thing and because it deals with teaching my classes!]

Switching Things Up, Need Help: Geometry is on the Horizon

For the past seven years, since I started teaching, I have been teaching calculus. When I started, I had 8 students in my class. Now I have 36 students. I’ve had to shift how I’ve thought about the course tremendously, and I’ve undergone a dramatic transformation in the content I teach (it is non-AP) and in the style in which I teach it. Seven years with a class is both a blessing and a curse. And honestly right now, I’ve reached the end of my usefulness for the course. I’m spinning my wheels. The only way I would be able to do a better job with it is to leave it for a few years and come back to it with a fresh pair of eyes.

And luckily, I have the opportunity to try something new. Next year, I will be giving up Calculus to teach an Advanced Geometry course for the first time. In fact, it’s the first time I’ll have ever taught geometry at all.


When I first began teaching, I was scared of geometry. Partly because as a student in high school, I found geometry to be uninteresting. It certainly didn’t have the elegance of algebra, at least the way I was taught it. Partly because I realized in that course — more than any other course — you as a teacher really have to focus on hard things. If you want kids to be able to do a proof of any kind (two-column or not), you are really teaching intuition building and connection making. Which is tough, and daunting for any new teacher, and this is why I recoiled at the thought. Now, years later, I see this as such an exciting challenge.

Right now, I am not anywhere about how to teach this course. And in fact, I’m only teaching one section and the other teacher is teaching three sections. But he’s very open to really revitalizing the course. So now we’re in exciting territory. Before I go bananas on scouring everything out there, I thought I’d crowdsource.

For any of you geometry teachers out there, if you have time to answer one or two (or all!) of these questions in the comments, I’d be ever so grateful!

1) What are your favorite geometry teaching resources — both online and offline? I’m talking books, websites, applets, manipulatives, whaever?

2) What are your favorite math teacher blogs that focus on geometry?

3) Is there a lesson you absolutely could not imagine teaching Geometry without?

4) Do you teach the course with a connective thread? Like: We are studying space and the properties inherent in space as we build space? Or: We are studying exactitude –and in particular, how we define mathematical entities so they yield uniquely understandable creatures? Or: We are studying “measurement” (in the vein of Paul Lockhart’s book).

5) I’m concerned that our kids lose a lot of their Algebra I skills when they take geometry. The other teacher and I have talked about putting coordinate geometry front and center from the beginning to help with this. Do y’all do anything else that helps keep their algebraic skills sharp, and maybe even push them forward?

6) Anything else? Problem solving? Sangakus? Geogebra use? Things you throw out because you feel strongly it’s only taught because it’s always been taught? Incorporation of Euclid’s Elements or math history? Graphic-design-y projects? Math art?

UPDATE: WOW, everyone, thank you so much for your resources and advice and for taking the time to type out so much great stuff. Now I’m genuinely THRILLED and CHOMPING AT THE BIT to get started re-learning geometry (and then teaching it). I am going to sort through things this summer!