  Many studies have found that neutron star mergers leave a fraction of the
stars' mass in bound orbits surrounding the resulting massive neutron star or
black hole. This mass is a site of $r-$ process nucleosynthesis and can
generate a wind that contributes to a kilonova. However, comparatively little
is known about the dynamics determining its mass or initial structure. Here we
begin to investigate these questions, starting with the origin of the disk
mass. Using tracer particle as well as discretized fluid data from numerical
simulations, we identify where in the neutron stars the debris came from, the
paths it takes in order to escape from the neutron stars' interiors, and the
times and locations at which its orbital properties diverge from those of
neighboring fluid elements that end up remaining in the merged neutron star.
