A number string is a set of related math problems, crafted to support students to construct big ideas about mathematics and build their own strategies (Fosnot & Dolk, 2002). Typically, a teacher gathers her students near the board and presents the problems — one at a time — providing plenty of time for students to think. Students first solve the problems mentally, then share their strategies with the class while making sure they understand others’ thinking. Meanwhile, the teacher is representing students’ strategies on a model and facilitating conversation among students about the sense they are making within and between the problems. Strings are not a rigid recipe but a flexible classroom routine — used by teachers daily or weekly with both small groups and entire classes.