The static and stable orientation of an object floating in water, such as a diver, is determined by its center of buoyancy and its center of mass. At stable equilibrium, they will be lined up by gravity and buoyancy with the center of buoyancy vertically above the center of mass. The diver's overall buoyancy and center of buoyancy can routinely be adjusted by altering the volume of the gas in the BC, lungs and diving suit. The diver's mass on a typical dive does not generally change by what seems like much (see above—a typical dive-resort "aluminum 80" tank at contains about of air or nitrox, of which about is typically used in a dive, although any air spaces such as in the BC and in diving suits will expand and shrink with depth pressure. Larger changes in buoyancy are possible if the diving weights are jettisoned, or a heavy object is picked up.