"Why have you written six times five?" Teachers' use of why in Norwegian mathematics classrooms

From classroom data consisting of transcripts from 20 videotaped lessons, we studied five Norwegian mathematics teachers’ use of the word why. It is our initial assumption that asking why in mathematics is a means to support students’ mathematical reasoning. We found evidence for this assumption in our data. However, the analysis of the 114 occurrences of why indicated that the most common purpose for asking why was to make students re-state thinking they had already done, thus not prompting new mathematical reasoning. Moreover, when teachers used why in questions not about mathematics, they usually did so in a rhetorical and reproachful manner. We discuss these ambiguities and implications for teaching.