  Despite the enormous theoretical and application interests, a fundamental
understanding of the glassy dynamics remains elusive. The static properties of
glassy and ordinary liquids are similar, but their dynamics are dramatically
different. What leads to this difference is the central puzzle of the field.
Even the primary defining glassy characteristics, their implications, and if
they are related to a single mechanism remain unclear. This lack of clarity is
a severe hindrance to theoretical progress. Here, we combine analytical
arguments and simulations of various systems in different dimensions and
address these questions. Our results suggest that the myriad of glassy features
are manifestations of two distinct mechanisms. Particle caging controls the
mean, and coexisting slow- and fast-moving regions govern the distribution of
particle displacements. All the other glassy characteristics are manifestations
of these two mechanisms; thus, the Fickian yet non-Gaussian nature of glassy
liquids is not surprising. Our results have crucial implications on how the
glassy dynamics data are analyzed, challenge some recent suggestions on the
mechanisms governing gassy dynamics, and impose strict constraints that a
correct theory of glasses must have.
