 According to the CM model, an explanation of some event E will trace the causal processes and interactions leading up to E (Salmon calls this the etiological aspect of the explanation), or at least some portion of these, as well as describing the processes and interactions that make up the event itself (the constitutive aspect of explanation). In this way, the explanation shows how E “fit[s] into a causal nexus”(1984: 9). For example, when two billiard balls collide (event E), the trajectory of each of the balls is a causal process (as shown by the fact that if the balls were scratched, such marks would persist) and their collision is a causal interaction. Explaining E will involve both tracing these trajectories and noting that E involves an interaction.