During professional development, the authors learned about strings--a powerful instructional routine used in elementary classrooms to elicit students' mental mathematics strategies and use those strategies to build conceptual understanding. Strings provided the authors with a way to make relatively small changes to guided notes in algebra 1, algebra 2, precalculus, and AP calculus classes. Over time, this led to profound changes in the students. They actively worked to solve problems in multiple ways without being shown what to do first; they used more precise mathematical language in their explanations; and they began to regularly describe and justify their thinking and question the reasoning of others. Finally, they gained a better grasp of the techniques the authors wanted them to learn and also why these techniques worked and how they were connected to underlying mathematical ideas. This article discusses what a string is, how strings are structured, designing and planning strings, and facilitating strings.