liu and yau recently introduced a new quasi-local mass in gr. it is a function on a 2-surface in a 4-manifold. they showed that it was positive. it is the maximum of the brown-york energy over all 3-slicings containing the given 2-surface. the liu-yau mass has unpleasant features, it looks much more like an energy than a mass. in particular it is unboundedly large on any solution of the einstein equations, including minkowski space! in spherical symmetry, however, it has a natural physical interpretation. consider a regular spherical 3-slice filling the interior of the given 2-slice. take the integral of the energy density of the interior. the liu-yau mass of the boundary is the minimum of this total energy content over all regular fillings. no other quasi-local mass gives such interior information.