Sum Stacks is a number-combination game where students build equations to match target numbers shown at the top of the board. Students begin by selecting a starting number, then choose additional blocks that include both an operator and a number to combine with it. As they progress, new operations are introduced—starting with addition, then adding subtraction, followed by multiplication, and eventually division, with decimals appearing in later levels. This gradual progression supports K–5 learners by strengthening number sense, fact fluency, and understanding of operations. Students practice seeing numbers flexibly, choosing efficient strategies, and reasoning about how different operations change values. Later levels deepen mathematical thinking by introducing constraints, such as blocks that can be used more than once, encouraging planning, repeated reasoning, and early algebraic thinking as students decide how to reach targets in multiple ways.