Standards documents, such as the Common Core State Standards for Mathematics and "Principles and Standards for School Mathematics", expect teachers to foster mathematics learning by engaging students in meaningful mathematical discourse to expose students to different ways of thinking about and solving problems and positively influence their problem-solving abilities, reasoning skills, and thinking processes. However, merely getting students to talk in math class is not enough to assure that learning will occur; once teachers get students to express their ideas, they must decide what to do with those ideas. Teachers' follow-up moves, such as evaluating, rebroadcasting, acknowledging, or making a related statement or question, will shape "the nature and flow of classroom discussions and the cognitive opportunities afforded to students". Thus, engaging students in meaningful mathematical discourse can be rather daunting because it requires teachers to make careful pedagogical choices and spontaneous decisions in the midst of the instructional process; and when students' ideas are considered, it can be difficult to predict and manage the direction of instruction. Nevertheless, a teacher's ability to listen effectively and respond appropriately to students' thinking has been shown to play a critical role in effective mathematics instruction. This article describes mathematical discourse that occurred over a two-week period during face-to-face class meetings and on an asynchronous, online discussion board (using Blackboard) and a teacher educator's efforts to listen and be responsive to preservice teachers' (PSTs) ideas about mathematics. The mathematical goal for the unit was to help the PSTs realize that the perimeter and area of different polygons are not always directly related. The course was structured such that the twenty-six female PSTs could earn participation points for engaging in in-class discussions or contributing to online discussions. Thus, the online discussion boards furnished an alternative for PSTs who may have been uncomfortable articulating their ideas during in-class discussions to still express the sense they were making of the mathematics. (Contains 2 figures.)