This article discusses an activity designed to encourage writing to learn in mathematics. There were three stages of data collection. An assessment, requiring basic algebra only, was completed by 118 undergraduates from statistics and calculus courses. Students were given summaries of all participant responses, along with the correct answers. After discussing the results, 25 students wrote about the underlying concepts needed to answer the questions correctly and 38 gave reasons someone might answer the questions incorrectly. The written responses were easy to evaluate and revealed much about misunderstandings. A follow-up survey was given to students to elicit their attitudes.