Much of the ethical debate about social robotics applications hinges on the ontological classification of our interactions with robots, but a detailed ontological account of simulated social interaction is still missing. In this paper I briefly explain why characterizations of human-robot interactions are best undertaken in a neutral, ‘technical’ idiom. Then I define five modes of simulation (functionally replicating, imitating, mimicking, displaying, and approximating) in terms of relations between processes. I sketch how these notions can be used to describe more precisely what a certain robot ‘can do.’ In conclusion I sketch a general strategy for developing a fine-grained taxonomy of social interactions.