Using a discourse analytic approach from the work of Hoey (1991) and a dual processing model from Wray (2000), this paper compares the language produced by the same classes of children when they are engaged in role-play and when they are playing rule-based games. We find that role-play tends to be richer in 'frozen' pair parts, where the responses are predictable, and that rule-based games are more conducive to dispreferred responses and bound exchanges. Overall, this means that role-plays appear to create 'short, fat' exchanges, while rule-based games generate 'tall, thin' ones. We argue the transition from discourse complexity to grammatical complexity demonstrates what Vygotsky (1978, 1987) called a zone of proximal development (ZPD), conceived of not as a mechanism for learning in general but rather as a specific link between microgenetic learning and ontogenetic development. Interpreting this cross-sectional view of the data ontogenetically not only provides an explanation for why role-play seems to be developmentally prior to rule-based games, but can also help explain how the intra-mental rules of grammar are precipitated from inter-mental relations in discourse. For children, foreign language learning allows a game-like inversion of first language acquisition processes, making rules explicit and discourse roles much less concrete.