The purpose of this activity is for students to revisit Stage 3 of the What's Behind My Back? center, which was first introduced in grade 1. Students begin with a tower of 10 connecting cubes. One student snaps the tower into 2 parts, puts 1 part behind their back, and shows the other part to their partner. Their partner records an addition equation with a blank to represent the missing part, figures out how many cubes are behind their back, then completes the equation. Throughout the activity, students practice using what they know about sums of 10 and the relationship between addition and subtraction to find unknown addends. Students explain their reasoning about how they know how many cubes are missing to their partner (MP3). As students discuss, listen for the ways students show how they are looking for and using the counting sequence, properties of operations, and the relationship between addition and subtraction (MP7).

• Ask a student to play a round of the game with the class.
• As the student displays the tower, write the related addition equation with an unknown addend on the board. (For example, if a student shows 2 blocks, write .) 
• “How does this equation represent the game?”
• “What number makes the equation true?”
• “How can we represent the game with a subtraction equation?” ( or )
• If needed, ask students to show how the numbers in the addition and subtraction equations represent the cubes in the round.

The purpose of the activity is for students to practice their fluency with sums of 10 as they find the numbers that make equations true. In this activity, students represent images of connecting cube towers with equations and match equations with an unknown number to these images (MP2). In the Activity Synthesis, students explain how their equations match the images (MP6).

• Display .
• “Which cube tower does this equation represent?” (C)
• “Where do you see each part of the equation in the tower? (There are 10 cubes altogether. 7 cubes are yellow and the unknown number of cubes are blue.)
• “How can the tower help us find the unknown number?” (It helps you see there are 10 blocks. 7 yellow and 3 blue make 10. If we take away the 7 yellow, we see the unknown number is 3.)