In recent years, teacher collaboration has emerged as an important strategy to drive improvement, informed by research showing how on-the-job interactions can boost teacher development and effectiveness. Schools across the United States are adjusting their professional cultures and workplace practices in response, creating formal opportunities for teachers to learn from one another and work together through shared planning periods, teacher leadership roles, and professional learning communities. Despite these changes, one constant often remains: legacy school buildings that follow an "egg crate" or cellular design, in which long hallways are lined with nearly identical, self-contained classrooms. This type of building, created to promote efficiencies and separate students into age-based groups to receive direct instruction by a single teacher, was not designed with adult collaboration in mind. The authors spent four years studying a midwestern suburban district, interviewing staff and analyzing building plans and survey data to explore this question. Their analysis finds that physical proximity predicts staff interactions, with teachers and school leaders more likely to interact about instruction with colleagues who are located physically close to them or with whom they are likely to cross paths during the school day. In addition, teachers and administrators often reference physical proximity in describing why and how they interact with one another, with chance encounters due to proximity serving as a supplement to more formal collaboration. Given these findings, schools and school districts should think carefully about where they assign school staff to workspaces, no matter their design.