This paper argues that the teaching of a subject can change our understanding of what it means to know, to teach and learn that subject. It also argues that when our understanding is questioned and changes then academic work can become an emotionally charged endeavour. This paper reports on a study where, over a semester's teaching, around two thirds of teachers changed some aspect of their scholarly thinking or practice. These teachers were teaching a range of first and second year classes, they were not new to teaching, nor were they unfamiliar with the teaching of the subject. For approximately one third, the change in understanding was not major. It involved a change in teaching practice but it did not involve the questioning of their existing understanding of subject matter. For another third, however, the change was substantial. These teachers, in some way, questioned previously taken for granted assumptions, they re-thought aspects of the structure of the discipline or the relationship of the subject to the discipline. They also revised their ideas about how to best teach that subject and what learning the subject involved. This change invariably involved anxiety and uncertainty and in some cases this was extreme. Little attention has been paid to change in teachers' understanding of subject matter and little, or no, research has focused on the emotional impact of this change. This initial exploration of these neglected aspects of university teaching suggests a rich vein for further exploration. This paper builds on previous work that has used phenomenography to examine changes in university teachers' understanding of subject matter taught. An analysis of metaphor is used to explore change and to tap into the rich and complex emotional experiences that accompany this change.