Grounded in Vygotskyan sociocultural theory, this qualitative, classroom-based investigation incorporates an open-ended task framework. Open tasks in the present study were defined as ones designed to take shape in the second language classroom as teacher and adult learners jointly worked through exploratory problems. One central feature of open task discourse was found to be emerging foci, as learners attended to lexical, grammatical, and discourse structures that grew out of their surrounding interaction. Conversation analysis transcriptions provided evidence that these unplanned moments allowed participants to negotiate intersubjectivity and fostered the building of new knowledge in learners' zones of proximal development (ZPDs). Findings suggest that studying how students participate in open-ended communicative activity is critical to understanding how new meanings become socially constructed and learned. (Contains 3 figures and 3 tables.)