The introduction of problem-based learning into K-12 science classrooms faces the challenge of achieving the dual goal of learning science content and developing problem-solving skills. To overcome this content-process tension in science classrooms, we employed the knowledge-creation approach as a boundary object between the two seemingly contradicting activities: learning of science content and developing problem-solving skills. As part of a design research, we studied a group of Grade 9 students who were solving a problem related to the Law of Conservation of Energy. Through the lens of the activity theory, we found that students' understanding of the intended science knowledge deepened as they made sense of the disciplinary-content knowledge in the context of the problem and concurrently, the students successfully developed solutions for the problem. This study shows that developing problem-solving competencies and content learning need not be disparate activities. On the contrary, we can harness the interdependency of these two activities to achieve dual goals in learning.