User interfaces typically feature tools to act on objects and rely on the ability of users to discover or learn how to interact with them. Previous work in HCI has used the Theory of Affordances to explain how users understand the possibilities for action in digital environments. A complementary theory from cognitive neuroscience, Technical Reasoning, posits that users accumulate abstract knowledge of object properties and technical principles known as mechanical knowledge, essential in tool use. Drawing from this theory, we introduce interaction knowledge as the "mechanical" knowledge of digital environments. We provide evidence of its relevance by reporting on an experiment where participants performed tasks in a digital environment with ambiguous possibilities for interaction. We analyze how interaction knowledge was transferred across two digital domains, text editing and graphical editing, and conclude that interaction knowledge models an essential type of knowledge for interacting in the digital world. CCS CONCEPTS • Human-centered computing → HCI theory, concepts and models.