The key to understanding the development of student misconceptions is to ask students to explain their thinking. Time constraints of classroom teaching make it difficult to consult with each and every individual student about their thought processes. However, when a particular error keeps surfacing, simply marking the response as incorrect will not assist the student. A two-minute conversation can help a student's mathematical understanding. In this article, the author describes a misconception about decimal place value she identified from the examination response of a Year 7 student. (Contains 3 figures.)