# Clip 17/18: Debrief: Classifying Triangles Part 5

## Overview Download Transcript (PDF)

David Foster, mathematics education leader and mentor, provides an analysis of Hillary Lewis’ fourth- grade lesson on classifying triangles. Foster highlights components of the Teaching for Robust Understanding (TRU) framework, the mathematical practices embedded in the lesson, and the idea of measurement and precision.

He discusses the cognitive demand placed on the students, making connections to the Van Hiele model for learning mathematics.

## Teacher Commentary

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Hillary Lewis

One of the difficult things that I do love about lesson study is that we don't know what we're going to learn from it. So often in teaching we know, or we think we know what we want our students to learn. But with lesson study we don't always know what we're going to learn.

We have our goals, and things we're going to try and see if they work, but it's kind of like a science experiment in some ways. You go in and you try something and see how it works, and tweak it a little bit, and try it again.

You don't know what's going to work, or what you're going to learn from it from the get go. In the beginning, that can feel challenging, because I tend to be kind of a black-and-white thinker; I want to know where my end goal

is.