One of the most vital decisions that teachers make during classroom discussion is whether to and how to validate the thinking that students present to the class. In this thesis I describe a study that addressed the issues that are associated with the decisions that teachers make in regards to validating students' ideas. Through qualitative research I explored these issues through videotaping an expert teacher, taking field notes, and conducting interviews. I share a description of what it looks like for one middle school mathematics teacher to make different decisions to use student input during class. The expert teacher in this study chose to use student input more than any other decision that she could have made. This study can help pre-service teachers and teachers learn to use student input.