  The conformal transformation of the Misner-Sharp mass is reexamined. It has
recently been found that this mass does not transform like usual masses do
under conformal mappings of spacetime. We show that when it comes to conformal
transformations, the widely used geometric definition of the Misner-Sharp mass
is fundamentally different from the original conception of the latter. Indeed,
when working within the full hydrodynamic setup that gave rise to that mass,
i.e. the physics of gravitational collapse, the familiar conformal
transformation of a usual mass is recovered. The case of scalar-tensor theories
of gravity is also examined.
