The "Next Generation Science Standards" provide opportunities for students to experience the link between science and engineering. In the December 2011 issue of "Science and Children," Rodger Bybee explains: "The relationship between science and engineering practices is one of complementarity. Given the inclusion of engineering in the science standards and an understanding of the difference in aims, the practices complement one another and should be mutually reinforcing in curricula and instruction" (p. 15). This month's column explores this complementary nature of science and engineering in the elementary classroom. While engineering practices can be used effectively to help elementary students develop scientific knowledge, scientific knowledge gained through use of the scientific practices can also be used to solve engineering problems. The example used in this column primarily addresses one aspect of this duality by focusing on how formative assessment can be used to assess students' readiness to define and carry out an engineering problem using knowledge gained through use of the scientific practices. Students plan and carry out a scientific investigation of pendulums using simple materials such as string and washers, teachers should observe and listen closely for evidence that students can perform the practice of planning and carrying out an investigation.