This column provides ideas and techniques to enhance your science teaching. This month's issue discusses how children think about the small-particle model of matter. What Richard Feynman referred to as the "atomic hypothesis" is perhaps more familiar to us as the small-particle model of matter. In its most basic form, the model states that all matter is made of particles that are too small to see, which have empty space between them and are in constant, random motion. As Feynman suggests, an understanding of this model equips students to explain a broad range of phenomena. The "Next Generation Science Standards" ("NGSS") echo this sentiment. Until instructional materials catch up with the "NGSS," teachers will have to rely on their own resources even more than usual to design instruction. A critical step in any instructional design process, regardless of the topic, is developing a firm grasp of how students are likely to think about the content. So, in considering the small-particle model, how do fifth-grade students begin to understand how matter is structured? What ideas are they likely to bring into the classroom? How can teachers assess student thinking? This article addresses these important questions and provides practical guidance.