This article sketches a theoretical framework that I have been devising in order to teach the nature of science (NOS) to pre- and in-service science teachers for all the educational levels (from kindergarten to university). The framework suggests that teachers should learn a set of simple ‘key-ideas’ from NOS. Those key-ideas can be selected and validated through a matrix that has two ‘structuring’ components: 1. a list of the main schools of twentieth-century philosophy of science (called stages); and 2. a list of their main fields of inquiry (called strands). Examples of NOS contents within and across some of the three stages and seven strands that I have identified, together with suitable pedagogies, are presented here under the form of three didactical units.