The purpose of this activity is for students to learn Stage 1 of the What’s Behind My Back? center. Students find multiple decompositions of a number. Students begin with a tower of connecting cubes. One student breaks the tower into 2 parts and shows the parts to their partner. Their partner finds and states the number of cubes in each of the 2 parts. Each partner represents the 2 parts on their recording sheet by coloring the connecting cubes and writing an expression. It may be helpful to give each group of students a single color of connecting cubes so the color of the cubes doesn’t distract students from identifying the two parts. In future stages of this center, students hide cubes behind their back after snapping a tower. If needed, you can choose not to use the name What’s Behind My Back? when playing this stage or let students know they will do that in future stages of this center. 

In this lesson, students break a tower of 8 cubes. The recording sheet is printed in the student book for this activity. There is a blackline master available for students to begin with towers of 5–10 connecting cubes during centers in future activities and lessons.

When students write an expression to match the connecting cubes, they reason abstractly and quantitatively (MP2).

• “What did you notice each time you snapped the tower?” (There were always 8 cubes. The 2 parts that we broke the tower into were different each time.)
• “What are the different ways that you and your partner found to break the tower of 8 cubes into 2 parts?” 
• Display a tower of 2 and a tower of 6.
• “What expression can we write to represent the 2 parts of this tower?” ( )

The purpose of this activity is for students to decompose numbers in more than one way. This is the first time that students begin with written numbers rather than a group of objects. Students may use objects, such as connecting cubes, to represent each number and find different ways to decompose the number into two parts. 

• “Which tool(s) did you use to help you find different ways to break apart numbers into two parts? How did each tool help you?”

The purpose of this activity is for students to choose from activities that offer practice with addition and subtraction.

Students choose any previously introduced stage from these centers:

• What’s Behind My Back?
• Check it Off
• Find the Value of Expressions
• Bingo
• Shake and Spill

• “Tell your partner one thing you noticed about how they worked in centers today.”