The educational content of a technical topic consists, ultimately, of elementary chunks of knowledge. Identifying and classifying such units -- Testable, Reusable Units of Cognition, or "Trucs" -- serves to understand the topic better, and can be useful for teaching a class on the topic, writing a textbook or course notes, defining a standard curriculum, preparing exam questions, assessing job candidates' claims that they master the topic, and in general to compare and consolidate educators' understanding of the area. Trucs are similar to "design patterns", but applied to education. This article defines the notion of Truc, explores its applications, and introduces a standard approach for specifying and classifying the Trucs making up an area of knowledge. 1 Context When "those who can" [2] or even "those who can teach" develop some interest in pedagogical techniques, they are likely to be less concerned with theories of knowledge acqui...