Category: classroomaction

Total 43 Posts

You Can’t Break Math

We were solving linear equations in Ms. Warburton’s eighth grade class last week and I learned (or re-learned, or learned at greater depth) a couple of truths about mathematics.

As I approached B and R, I misread them as disengaged. In fact, they were thinking really, really hard about this beast:

B suggested they multiply by two as a “fraction buster.”

(One small pleasure of guest teaching is trying to identify and decode the vernacular of each new class. I heard “fraction buster” more than once.)

R asked, “But do we multiply this by two or the whole thing by two?”

If you’ve taught math for a single day, you know the choice here.

You can tell them, “You multiply the whole thing by two.” That’d be helpful by the definition of “helpful” that includes “completing as many math problems as correctly and quickly as possible.” But in terms of classroom management, I’ll be doing myself no favors, having trained B and R to call me over whenever they have any similar questions. More importantly, I’ll have done their relationship with mathematics no favors either, having trained them to think of math as something that can’t be made sense of without an adult around.

Variables like x and y behave just like numbers like -2 and 3.” I said. I wrote this down and said, “Try out both of your ideas on this version and see what happens.”

After some quick arithmetic, they experienced a moment of clarity.

In the next class, students were helping me solve 2x – 14 = 4 – 2x at the board. M told me to add 2x to both sides. One advantage of my recent sabbatical from classroom teaching is that I am more empathetic towards students who don’t understand what we’re doing here and who think adding 2x to both sides is some kind of magical incantation that only weird or privileged kids understand.

So at the board I was asking myself, “Why are we adding 2x to both sides? What if we added a different thing?”

Then I asked the students, “What would happen if we added 5x to both sides? What would break?”

Nothing. We decided that nothing would break if we added 5x to both sides. It wouldn’t be as helpful as adding 2x, but math isn’t fragile. You can’t break math.


  • I haven’t found a way to generate these kinds of insights about math without surrounding myself with people learning math for the first time.
  • One of my most enduring shortcomings as a teacher is how much I plan and revise those plans, even if the lesson I have on file will suffice. I’ll chase a scintilla of an improvement for hours, which was never sustainable. I spent most of the previous day prepping this Desmos activity. We used 10% of it.
  • Language from the day that I’m still pondering: “We cancel the 2x’s because we want to get x by itself.” I’m trying to decide if those italicized expressions contribute to a student’s understanding of large ideas about mathematics or of small ideas about solving a particular kind of equation.

Featured Comments

Here is an awesome sequence of comments in which people savage the term “cancel,” then temper themselves a bit, and then realize that their replacement terms are similarly limited:

Corey Andreasen:

I have a huge problem with ‘cancel.’ It’s mathematical slang, and I’m OK with its use among people who really understand the mathematics. But among learners it obscures the mathematics and leads to things like “cancelling terms” in rational expressions.

Melissa Lechleiter:

I think the word “cancel” is misused in math when teaching students. We are not canceling anything we are making ones and zeros.


I never say cancel. I’ve worked hard to eliminate it from any teaching I do. Same with cross multiply, never say it. Instead I say “add to make zero” or “divide by or multiply by the reciprocal of to make one”.

Sam Shah:

We use “cancel” to mean too many things and so they use the term anytime they want to get rid of something or slash something out. The basis for my concern: when I ask kids “why can you do that?” they often can’t explain.

Jeremy Hansuvadha:

However, when it comes to squaring a square root, what is most accurate to say? I don’t correct the kids in that case, and I tell them that cancel means the same thing as “undo”.

Paul Hartzer:

If the point is to be rigorous, “apply the inverse” is more rigorous and technical than “cancel”.

More miscellaneous wisdom on language in mathematics:

David Butler:

In the topic of “get x by itself”, I’ve started saying “We want to say what x is. What would that look like?” They usually say “It would look like x = something”. Then they’ve chosen what the final equation ought to look like for themselves.

I wonder what would happen if we had an equation and then asked them to find out what, say, 2x+1 was.

Joel Penne:

Even in my Advanced Algebra 2 classes I have started using the phrases “legal” and “useful”. In the original post adding 5x to both sides was definitely “legal” just not as “useful”.

Laura Hawkins:

One of my refrains is that an algebraic step is “correct, but not useful.” Inspired by my dance teacher, I also talk about how a particular procedure is lovely, just not in today’s choreography (which is geared towards solving for an unknown/simplifying a trig expression/ finding intercepts, etc).

Great Classroom Action

Tracy Zager illustrates a key feature of some of my favorite math tasks: their constraints are simple, but they create paths for complex thinking and ever more interesting questions:

I think my name is worth $239. Beat me? Haven’t figured out my $100 strategy yet.

Lisa Bejarano is a recipient of our nation’s highest honor for math teachers, so when she admits “I have no idea what I am doing” and starts sketching out a blueprint for great classrooms, I tune in:

Now, beginning with the first day of school, I intentionally work at building a unique relationship with each student. I make sure to find reasons to genuinely value each of them. This starts with weekly “How is it going?” type questions on their warm up sheets and continues by using their mistakes on “Find the flub Friday” and through feedback quizzes. I also share a lot of myself with them. When we understand each other, my classes are more productive. I still make plans, but I allow flexibility to meet my students where they are.

David Cox describes “a difficult thing for students to believe”:

Once students begin to believe that the way they see something is the currency, then our job is to simply help them refine their communication so their audience can understand them. Only then does the syntax of mathematics matter.

“Help me understand you.”

“Help me see what you see.”

Kevin Hall thoughtfully deconstructs his attempts to teach linear function for meaning, and includes this gem:

Once you introduce the slope formula, slope becomes that formula. It barely even matters if today’s lesson created a nice footpath in students’ brains between “slope” and the change in one quantity per unit of change in another. Once that formula comes out, your measly footpath is no competition for the 8-lane highway that’s opened up between “slope” and (y2—y1)/(x2­-x1).

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Great Classroom Action


Tricia Poulin makes some awesome moves in her #bottleflipping lesson, including this one:

Okay, so now the kicker: will this ratio be maintained no matter the size of the bottle?

Graham Fletcher offers us video of kindergarten students interacting in a 3-Act modeling task:

It’s always great to engage the youngins’ in 3-Act Tasks. I’ve heard colleagues say, “I don’t have time to do these types of lessons.” I hope this helps show that we don’t have time to not have the time.

Wendy Menard offers her own spin on the Money Duck, one of my favorite examples of expected value in the wild:

The students designed their own “Money Animals”, complete with a price, distribution, and an expected value. This was all done on one sheet; the design, price and distribution were visible to all, while the calculations were on the back. After everyone had finished, we had our Money Animal Bonanza.

Sarah Carter hosts the Mini-Metric Olympics, a series of data collection & analysis events with names like “Left-Handed Sponge Squeeze” and “Paper Plate Discus”:

After the measurements were all taken, we calculated our error for each event. One student insisted that she would do better if we calculated percent error instead, so we did that too to check and see if she was right. In the future, I think I would add a “percent error” column to the score tracking sheet.

Great Classroom Action


A. O. Fradkin used her students as manipulatives in a game of addends:

The classic mistake was for kids to forget to count themselves. Then I would ask them, “How many kids are not hiding under the blanket?” When they would say the number of kids they saw, I’d follow up with, “So you’re hiding under the blanket?” And then they’d laugh.

Cathy Yenca put students to work once they finished their Desmos card sorts:

From here, it becomes a beautiful blur. Students continue to earn “expert” status and become “up for hire”, popping out of their seats to help a bud. At one point today, every struggling student had a proud one-on-one expert tutor, and I just stood there, scrolling through the teacher dashboard, with a silly grin on my face.

I’d love to know how we could employ experts without exacerbating status anxieties. Ideas?

Laurie Hailer offers a useful indicator of successful group work:

It looks like the past six weeks of having students sit in groups and emphasizing that they work together is possibly paying off. Today, instead of hearing, “I have a question,” I heard, “We have a question.”

David Sladkey switches from asking for questions to requiring questions:

My students were working independently on a few problem when I set the ground rules. I told my students that I was going to require them to ask a question when I was walking around to each person. I also said that if they did not have a math question, they could ask any other (appropriate) question that they liked. One way or another, they would have to ask me a question. It was amazing.

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I also have kids sign up to be an expert during group work, indicating that they’re open to taking questions from other students. Sometimes, after a really good small group conference, I’ll ask a student to sign up to be an expert.

Great Classroom Action

Jennifer Abel creates a promising variety of card sort activity:

Basically, after dealing the cards, the basic idea is for kids to pass one card to the left while at the same time receiving one card from the player to their right. The object of the game is to collect all cards with the same suit/type/category. Here are two examples that I recently created for next year.

Julie Morgan offers three sharp lesson-closing activities. My favorite is “Guess My Number”:

I choose a number between 1 and 1000 and write it on a piece of paper. Each group takes it in turns to ask me a questions about my number. The questions can vary from “is it even?” to “what do the digits add up to?” to “is it a palindrome?” (my classes know I like palindromes!) When a group thinks they have figured it out they write it down and bring it up to me. Each group is only allowed three attempts so cannot keep guessing randomly. I like this for emphasising mathematical knowledge such as multiples, primes, squares, etc.

Pam Rawson contributes to #LessonClose with both a flowchart that illustrates her thought process at the end of classes and then some example exit polls for both “content” and “process” objectives:

As a member of the Better Math Teaching Network, I had to come up with a plan — something in my practice that I can tweak, test, and adjust with ease. So, I decided to focus on class closure. Since I don’t have an actual process for this, I had to think intentionally about what I might be able to do. I created this process map.

Robert Kaplinsky offers the #ObserveMe challenge:

We can make the idea of peer observations commonplace. It’s time to take the first step.