  We consider a set of hard point particles distributed uniformly with a
specified density on the positive half-line and all initially at rest. The
particle masses alternate between two values, $m$ and $M$. The particles
interact via collisions that conserve energy and momentum. We study the cascade
of activity that results when the left-most particle is given a positive
velocity. At long times we find that this leads to two fascinating features in
the observed dynamics. First, in the bulk of the gas, a shock front develops
separating the cold gas from a thermalized region. The shock-front travels
sub-ballistically, with the bulk described by self-similar solutions of Euler
hydrodynamics. Second, there is a splash region formed by the recoiled
particles which move ballistically with negative velocities. The splash region
is completely non-hydrodynamic and we propose two conjectures for the long time
particle dynamics in this region. We provide a detailed analytic understanding
of these coexisting regimes. These are supported by the results of molecular
dynamics simulations.
