Facilitating mental computation discussion comes with its own set of challenges for researchers and teachers. It is important to be able to help capture the students' ideas, before they disappear and seem like little more than moments of magic! In enabling children to learn how numbers work, the current emphasis on mental computation is driven by the principles of constructive learning. It is here that questions of "how" and "why" become particularly important for both teachers and students. When students are working mentally, simple questions posed to an individual or a class of students, such as, "How did you work this out?" and, "Why does that work?" allow teachers to peer into the world of numbers and gain an understanding from the perspective of the students. It also encourages students to re-evaluate their own ideas when exposed to the thinking of their peers. Experiences that encourage discussion and learning are far removed from the emphasis on activities that focus on testing that have dominated mental computation in primary and secondary classrooms for so long. Many teachers rightly feel that testing activities of some sort still have a place in the classroom, for example, as a quick way of gauging what types of questions students are able to cope with. However, as the sole approach to teaching mental computation, they really are a thing of the past. In this article, the author provides questioning techniques to help teachers "capture" the mathematical ideas of children as they explain their mental computation strategies.