  The cage effect is generally invoked when discussing the delay in the decay
of time correlation functions of dense fluids. In an attempt to examine the
role of caging more closely we consider the spread of the displacement
distributions of Brownian particles. These distributions are necessarily biased
by the presence of neighbouring particles. Accommodation of this bias by those
neighbours conserves the displacement distribution locally and presents a
collective mechanism for exploring configuration space that is more efficient
than the intrinsic Brownian motion. Caging of some particles incurs, through
the impost of global conservation of the displacement distribution, a delayed,
non-local collective process. This non-locality compromises the efficiency with
which configuration space is explored. Both collective mechanisms incur delay
or stretching of time correlation functions, in particular the particle number
and flux densities. This paper identifies and distinguishes these mechanisms in
existing data from experiments and computer simulations on systems of particles
with hard sphere interactions.
