  We use a simple hard-core gas model to study the dynamics of small exploding
systems. The system is initially prepared in a thermalized state in a spherical
container and then allowed to expand freely into the vacuum. We follow the
expansion dynamics by recording the coordinates and velocities of all particles
until their last collision points (freeze-out). We have found that the entropy
per particle calculated for the ensemble of freeze-out points is very close to
the initial value. This is in apparent contradiction with the Joule experiment
in which the entropy grows when the gas expands irreversibly into a larger
volume.
