Change in mathematics teachers' practice is often characterized as something that takes time and sustained intervention. In this article, I present the results of research that highlights a different kind of change--a profound change that takes place very quickly. Based on the analysis of 42 cases of such rapid and profound change, I also present a disaggregation of this phenomenon into five distinct mechanisms of change, each one rapid and profound. This disaggregation shows that not all changes, even when outwardly similar, are the same.