The teacher should continually remind students of conjectures and mathematical arguments that they have developed as part of the shared classroom experience and that can be applied to further work. Teachers should look for opportunities for students to revise, expand, and update generalizations they have made as they develop new mathematical skills and knowledge. Matt's idea about tripling in Ms. Taylor's third-grade class could provide the basis for students to reason about a larger class of problems. Even students who seem to have developed a clear argument about a mathematical relationship need to be questioned and challenged when they are ready to encounter new aspects of the relationship. For example, a class of third graders had spent a great deal of time working with arrays in their study of multiplication. As a group, they were very sure that multiplication was commutative, and they could demonstrate this property using an area model. In the fourth grade, they began encountering larger numbers; when the teacher noticed that some students were using commutativity, she asked the class what they knew about it. At first they seemed certain that multiplication is commutative in all cases, but when she pressed, "But would it work for any numbers? How about 43 279 times 6 892?" they lost their confidence. They could no longer use physical models to show commutativity with such large numbers, and they needed further work to develop mental images and mathematical arguments based on what they had learned from the physical models. It is likely that these students will also need to revisit commutativity when they study computation with fractions and decimals.