  Recent progresses in single particle tracking have shown evidences of
non-Gaussian distribution of displacements in living cells, both near the
cellular membrane and inside the cytoskeleton. A similar behavior has also been
observed in granular media, turbulent flows, gels, and colloidal suspensions,
suggesting that this is a general feature of diffusion in complex media. A
possible interpretation of this phenomenon is that a tracer explores a medium
with spatio-temporal fluctuations which result in local changes of diffusivity.
We propose and investigate an ergodic, easily interpretable model, which
implements the concept of diffusing diffusivity. Depending on the parameters,
the distribution of displacements can be either flat or peaked at small
displacements with an exponential tail at large displacements. We show that the
distribution converges slowly to a Gaussian one. We calculate statistical
properties, derive the asymptotic behavior, and discuss some implications and
extensions.
