The purpose of this activity is for students to build any object they want. In the next activity, students focus only on rectangular prisms. As students count, they will need to make sure to count each unit cube once and only once. Monitor for these strategies:

• Touch each unit cube as it is counted.
• Count the unit cubes by layer (including when there are different numbers of unit cubes in each layer).
• Use addition or multiplication when the unit cubes are organized in layers.

• “How did you count the unit cubes?”
• “Where do you see unit cubes that can be counted as a group?”

• Discuss responses to the last problem.
• “Which objects were most challenging to count?” (Objects with weird shapes, so I had to count each individual unit cube.)
• “Which objects were the simplest to count?” (Organized shapes like towers, rectangles, cubes, or boxes.)
• Invite selected students to share the rectangular prisms they built and how they counted the number of unit cubes.
• “How did you count the unit cubes?” (There were 4 unit cubes in each layer and 2 layers, so that’s 8 unit cubes.)
• “We call these objects rectangular prisms. What were some ways you used to count the unit cubes in rectangular prisms?” (I counted the unit cubes in groups, and then skip-counted. I used multiplication to find the total.)

In the previous activity, students described how to count the number of unit cubes in an object and observed that for a special shape called a “rectangular prism,” the cubes are organized in a structure that makes counting them more systematic. The purpose of this activity is for students to continue to explore rectangular prisms, in particular their structure. As students describe the prisms, they communicate their ideas clearly and precisely (MP6). To describe an object requires students to identify key features of the object and communicate them in an unambiguous way to their partner.

This activity uses MLR2 Collect and Display. Advances: reading, writing.

• Invite partners who built different prisms to share their prisms and descriptions.
• “Was knowing the number of unit cubes in a layer and the number of layers enough information for you to understand the prism that your partner wanted you to build?” (My partner said there were 6 unit cubes on the bottom and 6 more on top. So I made a row of 6 unit cubes and put another row on top. But my partner had two rows of 3 instead of one row of 6.)
• Invite groups who built the same prisms to share.
• “How did you describe your prism to your partner?” (I said how many unit cubes were on the bottom and how tall the prism was.)
• Use this discussion to update the display, by adding (or removing) language, diagrams, or annotations.
• Display any prism made by a student.
• “How can we describe this prism, using language from our display?” (Highlight the use of language about “layers” or “stacks.”)