In the previous lesson, students used the Shake and Spill center to practice solving Put Together/Take Apart problems and describing the relationship between two parts and the total. In this lesson, students learn a new way to play Shake and Spill and consider ways to represent and solve Put Together/Take Apart, Addend Unknown problems.

As students play the center and solve related problems, they have the opportunity to apply the counting strategies they used with Add To, Change Unknown problems. This problem type may also elicit removing the known addend from the total, counting back, or other strategies students often use with problems that more explicitly elicit subtraction. As students notice and describe how they use these strategies, they take their first steps toward understanding subtraction as an unknown addend problem. Students will continue to explore subtraction as a way to find an unknown addend throughout the unit, so there is not a need to elicit this idea during the lesson if it does not come up naturally.