  Supercooled liquids display fascinating properties upon cooling such as the
emergence of dynamic length scales. Different models strongly vary with respect
to the choice of the elementary subsystems (CRR) as well as their mutual
coupling. Here we show via computer simulations of a glass former that both
ingredients can be identified via analysis of finite-size effects within the
continuous-time random walk framework. The CRR already contain complete
information about thermodynamics and diffusivity whereas the coupling
determines structural relaxation and the emergence of dynamic length scales.
