  We describe a simple classroom demonstration of a fluid-dynamic instability.
The demonstration requires only a bucket of water, a piece of string and some
used tealeaves or coffee grounds. We argue that the mechanism for the
instability, at least in its later stages, is two-dimensional barotropic
(shear-flow) instability and we present evidence in support of this. We show
results of an equivalent basic two-dimensional numerical non-linear model,
which simulates behavior comparable to that observed in the bucket
demonstration. Modified simulations show that the instability does not depend
on the curvature of the domain, but rather on the velocity profile.
