This study explores how teachers across the United States answer and reason on “teacher assessment tasks” designed to elicit content knowledge for teaching (CKT) about matter and its interactions. It leverages a think aloud approach, where participants were asked to reason about different assessment items that described teaching scenarios related to elementary science instruction about topics such as properties of matter, changes in matter, the particulate model of matter, and conservation of matter. Specifically, this study investigates: (1) To what extent do the participants use the intended knowledge and reasoning when responding to items designed to assess their CKT about matter and its interactions? and (2) When participants struggle to respond accurately to these items, what were their reasons for doing so? In particular, we were interested in examining the patterns in the nature of the knowledge they fail to leverage when responding to these CKT matter items. These presentation slides were prepared for the 2020 Annual International NARST Conference.