  We present an analysis of the mechanics of thin streams, which are formed
following the tidal disruption of cold, low-mass clusters in the potential of a
massive host galaxy. The analysis makes extensive use of action-angle
variables, in which the physics of stream formation and evolution is expressed
in a particularly simple form. We demonstrate the formation of streams by
considering examples in both spherical and flattened potentials, and we find
that the action-space structures formed in each take on a consistent and
characteristic shape. We demonstrate that tidal streams formed in realistic
galaxy potentials are poorly represented by single orbits, contrary to what is
often assumed. We further demonstrate that attempting to constrain the
parameters of the Galactic potential by fitting orbits to such streams can lead
to significant systematic error. However, we show that it is possible to
predict accurately the track of streams from simple models of the action-space
distribution of the disrupted cluster.
