Mathematical reasoning is a key proficiency in mathematics, however primary school teachers may have trouble noticing students’ mathematical reasoning mid-lesson. In this study, students’ mathematical reasoning has been analysed using video data collected as one grade of Year 5 and 6 students worked through the ‘Painted Cube’ task. Analysis reveals that students displayed a range of sophistication of mathematical reasoning. An ecological analysis of what student reasoning actions were visible to the teacher mid-lesson suggests that many of these actions were too subtle to be picked up by the teacher.