In the study I investigated the role of tools and task structures for science teachers of K-8 on the topic of balance in a professional development course. The findings show that the teachers tended to generate an additive rule, which is limited and contextualized, of balance condition, and failed to incorporate the more general multiplicative rule when thinking about balance. The study also shows that the teachers had difficulty in grasping a mathematical formula of balance condition. The learning outcome was partly a natural result of the materials the teachers used and the tasks they carried out. I conclude that learning tools and instructional tasks provide opportunities for understanding science content, but the understanding is confined to the tools and tasks employed.