Lesson 2

The purpose of this activity is to elicit ideas students have about the number 10. This routine provides an opportunity for all students to contribute to the conversation and for the teacher to listen to what knowledge students already have. When students share where they see 10 in the real-world, they show their ability to contextualize numbers (MP2). When students share about numbers that are close to 10 when counting, relate 10 ones to the unit ten, and sums and differences with the value of 10, they show what they know about the structure of whole numbers, place value, and the properties of operations (MP7).

• Display the number.
• “What do you know about 10?”
• 1 minute: quiet think time

• Record responses.

• If needed, “How could we represent the number 10?”

The purpose of this activity is for students to revisit stage 2 of the What's Behind My Back center, which was first introduced in grade 1. Students begin with a tower of 10 connecting cubes. They break apart the tower and represent the two parts with a drawing and an expression. 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, or the relationship between addition and subtraction (MP7).

• Gather a tower of 10 connecting cubes to use in the launch.

• Groups of 2
• Give each group 10 connecting cubes.
• “We are going to play a game you may have played in first grade called What’s Behind My Back?”
• Show students a tower of 10 cubes. Put the tower behind your back. Break off 4 cubes, and display the rest of the tower.
• “How many cubes are behind my back? How do you know?” (4 because there are 6 and 4 more makes 10. I know because \(10 - 4 = 6\).)
• 30 seconds: quiet think time
• 1 minute: partner discussion
• Share responses.
• “After one partner has hidden the cubes, switch roles.”

• 10 minutes: partner work time

• 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 \(2+\underline{\hspace{.5 cm}}=10\).) 
• “How does this equation represent the game?”
• “What number makes the equation true?”
• “How can we represent the game with a subtraction equation?” (\(10 - 2 = 8\) or \(10 - 8 = 2\))
• 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 synthesis, students explain how their equations match the images (MP6).

• Groups of 2
• “Look at cube tower A in your book.”
• “Write as many equations as you can think of that represent the tower.”
• 2 minutes: partner work time
• Share responses.

• “Continue writing equations for each cube tower. Write as many equations as you can think of for each tower. You can write addition and subtraction equations.”
• 5 minutes: partner work time
• “Now look at the equations in the table. Some of the equations might match the equations you wrote under the cube towers.”
• “Find the number that makes each equation true. Then write the letter of the cube tower that represents the equation.”
• 5 minutes: partner work time

• Display \(10 -7=\underline{\hspace{1 cm}}\)   
• “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 and 7 yellow and 3 blue would make 10. If we take away the 7 yellow, we see the unknown number is 3.)