During the past decades, notable improvements have been achieved in the
understanding of static and dynamic properties of granular materials, giving
rise to appealing new concepts like jamming, force chains, non-local rheology
or the inertial number. The `saltcellar' can be seen as a canonical example of
the characteristic features displayed by granular materials: an apparently
smooth flow is interrupted by the formation of a mesoscopic structure (arch)
above the outlet that causes a quick dissipation of all the kinetic energy
within the system. In this manuscript, I will give an overview of this field
paying special attention to the features of statistical distributions appearing
in the clogging and unclogging processes. These distributions are essential to
understand the problem and allow subsequent study of topics such as the
influence of particle shape, the structure of the clogging arches and the
possible existence of a critical outlet size above which the outpouring will
never stop. I shall finally offer some hints about general ideas that can be
explored in the next few years.Comment: 13 pages, 7 figure
