An air parcel does not have a rigid boundary, but we do not allow anything to pass through it. The boundary can move outward and inward, allowing the parcel to expand and compress.

Since no air can enter or leave the parcel, the air mass inside is constant. When the parcel expands, its density decreases (constant mass, increasing volume); when the parcel compresses, its density increases.

The force of gravity varies with the mass of the object. The upward pressure force, which results from the greater pressure from below against the smaller pressure from above (recall that atmospheric pressure decreases with altitude) depends on the volume of the parcel (the bigger it is, the larger the upward force). Note that density relates the mass of an object to its volume. We will find that the buoyancy force, which is [upward pressure force] minus [gravity force or weight], will have some correlation to density, or rather the difference in density between the object and the surrounding air.