Mental computation--that is, calculating in the head--is a relatively new topic in mathematics curricula for primary-age children. It is an important skill because it enables children to learn more deeply how numbers work, make decisions about procedures, and create strategies for calculating, thus promoting number sense--a well-developed understanding of number. When teaching mental computation, the emphasis should be on children developing their own strategies by exploring, discussing, and justifying their thinking and solutions. For some teachers, this requires a shift in beliefs and attitudes about what and how to teach mathematics in primary grades. Teachers need support to make these shifts. With this in mind, the author worked with Pam and Sue, two primary grade teachers, to help them improve their practice and introduce this new topic--mental computation--in their classrooms. Pam and Sue taught 6-, 7-, and 8-year-olds at a Brisbane (Australia) elementary school. This article describes how these teachers engaged children in developing strategic thinking skills in math, applying research to their classroom practice in order to do so.