In previous grades, students learned to find the number that makes 10 when added to a given number and to make a ten to add within 20 and 100. The ability to look for and use sums that have a value of 10 is a foundational skill for the grade 2 benchmark of fluency within 20 and for adding and subtracting within 1,000 using methods based on place value.

In this lesson, students revisit the What's Behind My Back? center. Students use connecting cubes to elicit their understanding of sums that have a value of 10 and the relationship between addition and subtraction. Students continue to practice recognizing and using these sums in lesson activities throughout the unit.

Tell students they will have a chance to revise their math community ideas at the end of this lesson. As they work today they should think about actions that may be missing from the current list.