The higher education sector has been working with threshold concepts since they were first postulated in 2003. Threshold concepts offer a way to focus on areas of content which students find challenging to master. Once mastered, threshold concepts explain and integrate further areas of learning. In this sense threshold concepts can be regarded as liminal. Although well established in the higher education sector, the use of threshold concepts has not been extensively explored in schools. This article focuses on Science in the upper secondary school. Yet it is possible to imagine scenarios where highly targeted teaching of liminal content leads to space being created for practical enquiry. There is an established theoretical pedagogical framework within which threshold concepts can sit comfortably and will be familiar to schoolteachers. This framework also provides a route by which troublesome liminal knowledge can be taught. There is then the question of defining and identifying threshold concepts. Threshold concepts do suffer from a lack of definition. Much of the literature explores ideas as diverse as complex physics and attitudes and values of nursery workers. This diversity has led to a discipline specific approach to defining concepts. It is argued that teachers can work with a range of stakeholders to identify troublesome knowledge. This could alter curriculum planning, particularly time allocation, to specific troublesome content and provide time for a more diverse learning experience for pupils.