The Virtual Math Teams (VMT) project investigates the innovative use of online collaborative environments to support effective mathematical problem-solving by small groups of learners in an online community. A key research issue for us is to understand how team interactions evolve over time and how to support them effectively. As designers, we expect the persistent records of the teams? interactions provided in the VMT environment to contribute to the sustainability of the teams? knowledge work. Our analysis of data collected from distributed virtual teams working on open-ended mathematical tasks across multiple sessions shows the situated usage of such persistent artifacts. In particular, we observe a series of "bridging" methods used to co-construct mathematical knowledge over time, evolve a sense of collectivity, and interlink the online environment with other interaction spaces. We propose that bridging, the purposeful crossing of interactional boundaries, is a consequential and often unsupported aspect of the collaborative user experience.