Successful embryogenesis requires proper sorting and compartmentalization of different cell types. Mechanical interactions between cells help govern these processes. In the past, physics-based theories have guided in vitro studies of cell sorting and tissue surface tension. Recent experiments have challenged this approach, indicating that adhesive molecules also act as signaling molecules that initiate local reorganization of actomyosin and demonstrating that cells at the boundary of a colony of initially identical cells become "mechanically polarized." Extending physical models to account for mechanical polarization helps solve a long-standing paradox about magnitudes of tissue surface tensions and potentially explains discrepancies between recent in vivo and in vitro cell-sorting experiments. New experiments are needed to further explore the connection between mechanical polarization and tissue boundary formation in vivo.