  A quasi-local mass has been a long sought after quantity in general
relativity. A recent candidate has been the Liu-Yau mass. One can show that the
Liu-Yau mass of any two-surface is the maximum of the Brown-York energy for
that two-surface. This means that it has significant disadvantages as a mass.
It is much better interpreted as an energy and I will show one way of doing so.
The Liu-Yau mass is especially interesting in spherical geometries, where mass
and energy are indistinguishable. For a spherical two-surface, it equals the
minimum of the amount of energy at rest that one needs to put inside the
two-surface to generate the given surface geometry. Thus it gives interesting
information about the interior, something no other mass or energy function
does.
