The day-to-day business of being a science or mathematics teacher involves the continuous assessment of students. This, in turn, is an inherently discursive process. The aim of the present study is to examine some of the specific discursive practices through which science and mathematics knowing is jointly produced through classroom interaction. In particular, we examine the coproduced nature of two students' not knowing--one in an outdoor elementary school science lesson and the other in an elementary school mathematics lesson. Our analysis is based on ideas in discursive psychology and challenges conventional interpretations of students' academic performance in school science and mathematics.