Eliciting, responding to, and advancing students' mathematical thinking all lie at the heart of great teaching. In this article, the authors describe a formative assessment approach that teachers can use to learn more about their students' mathematical thinking and inform their instructional decisions. This assessment approach draws on a widely known set of frameworks for children's thinking, Cognitively Guided Instruction (CGI) (Carpenter et al. 2014). It enables teachers to learn from students by giving them time to voice their understandings and confusions. By listening carefully, teachers convey to students that their experience matters. The primary purpose of assessment is to inform and improve the teaching and learning of mathematics. The assessment approach described in this article gives teachers opportunities to talk with students about their mathematical thinking and gain a better understanding of their students as mathematicians.