In this article a thermodynamic view of learning is taken. Internalisation of a threshold concept, the so-called act of “passing through the portal”, is viewed as a phase change. It is postulated that threshold concepts are associated with learning that both involves a large entropy change and that has competing possible learning outcomes. Through analogy with enthalpy of matter, it is predicted that certain learning activities will aid students, especially in the case of reaching true understanding of threshold concepts, much as certain physical processes aid the formation of diamond in preference to graphite.