 The relevant notion of interaction has been isolated as the result of contributions that come from widely different research areas intensively investigated only in relatively recent years, notably linear logic (Girard 1987) and the theory of concurrent processes. It is in these areas that a notion of composition as interaction of modules takes shape. We give here just a simple example where the composition of modules in the form of “parallel composition + hiding” is found in nature, in order to connect it with the origin of this idea in the semantics of concurrent processes developed by Hoare (1985), and also to afford a first glimpse into a simplified game formalism.