  We present an alternative explanation for the nature of turbulence in
molecular clouds. Often associated with classical models of turbulence, we
instead interpret the observed gas dynamics as random motions, induced when
clumpy gas is subject to a shock. From simulations of shocks, we show that a
supersonic velocity dispersion occurs in the shocked gas provided the initial
distribution of gas is sufficiently non-uniform. We investigate the velocity
size-scale relation $\sigma \propto r^{\alpha}$ for simulations of clumpy and
fractal gas, and show that clumpy shocks can produce realistic velocity
size-scale relations with mean $\alpha \thicksim 0.5$. For a fractal
distribution, with a fractal dimension of 2.2 similar to what is observed in
the ISM, we find $\sigma \propto r^{0.4}$. The form of the velocity size-scale
relation can be understood as due to mass loading, i.e. the post-shock velocity
of the gas is determined by the amount of mass encountered as the gas enters
the shock. We support this hypothesis with analytical calculations of the
velocity dispersion relation for different initial distributions.
  A prediction of this model is that the line-of sight velocity dispersion
should depend on the angle at which the shocked gas is viewed.
