As computer-science-for-all initiatives continue to grow, many schools are looking for ways to introduce computer science skills and thinking to elementary-age children. Some initiatives have focused on coding as its own endeavor, not integrated with other subjects like mathematics, science, or literacy. Increasingly, developers and researchers are exploring ways that programming can be integrated into core subjects, although challenges remain to ensure they are mutually supportive of both subjects. The authors, a team of teachers and researchers, are investigating one such approach that integrates programming and elementary school mathematics, developing microworlds for Grades 2-5 that treat programming as a language to help children express and investigate mathematical ideas. In this article, the authors describe one of the programming environments they have developed, Number Line, and their observations of second graders' experiences and mathematical thinking while using this tool.