  We investigate the spreading of passive tracers in closed basins. If the
characteristic length scale of the Eulerian velocities is not very small
compared with the size of the basin the usual diffusion coefficient does not
give any relevant information about the mechanism of spreading.
  We introduce a finite size characteristic time $\tau(\delta)$ which describes
the diffusive process at scale $\delta$. When $\delta$ is small compared with
the typical length of the velocity field one has $\tau(\delta) \sim
\lambda^{-1}$, where $\lambda$ is the maximum Lyapunov exponent of the
Lagrangian motion. At large $\delta$ the behavior of $\tau(\delta)$ depends on
the details of the system, in particular the presence of boundaries, and in
this limit we have found a universal behavior for a large class of system under
rather general hypothesis.
  The method of working at fixed scale $\delta$ makes more physical sense than
the traditional way of looking at the relative diffusion at fixed delay times.
This technique is displayed in a series of numerical experiments in simple
flows.
