In this article, the authors illustrate how the practice of number strings--used regularly in a classroom community--can simultaneously support computational fluency and building conceptual understanding. Specifically, the authors will demonstrate how a lesson about multi-digit addition (CCSSM 2NBT.B.5) can simultaneously serve as an invitation to look for and make use of structure (SMP 7) and engage in mathematical reasoning (SMP 2). The authors present the work of second-grade teacher and co-author Dina Williams and her students. Williams teaches in a school in which all students are African-American or Latino/a and almost all students are from low-income households. In addition, several students with disabilities are included in her class. An advocate of equity in mathematics, Williams aims to engage all her students in rigorous mathematics. She supports their engagement through classroom routines--in this case, daily number strings. Within this routine, she shifts mathematical authority from herself to her students, who begin to see themselves as innovators of mathematical strategies. She makes strategies explicit by naming them, which helps her students make sense of them and begin to use them. With the number string, her goal is to support her students to think flexibly and strategically about addition, with the support of the open number line as a model. (A bibliography is included.)