Students at the junior high school (JHS) level often cannot use their knowledge of physics for explaining and predicting phenomena. We claim that this difficulty stems from the fact that explanations are multi-step reasoning tasks, and students often lack the qualitative problem-solving strategies needed to guide them. This article describes a new instructional approach for teaching mechanics at the JHS level that explicitly teaches such a strategy. The strategy involves easy to use visual representations and leads from characterizing the system in terms of interactions to the design of free-body force diagrams. These diagrams are used for explaining and predicting phenomena based on Newton's laws. The findings show that 9th grade students who studied by the approach advanced significantly from pretests to post-tests on items of the Force Concept Inventory--FCI and on other items examining specific basic and complex understanding performances. These items focused on the major learning goals of the program. In the post-tests the JHS students performed on the FCI items better than advanced high-school and college students. In addition, interviews conducted before, during, and after instruction indicated that the students had an improved ability to explain and predict phenomena using physics ideas and that they showed retention after 6 months. (Contains 8 figures and 6 tables.)