If you're working with a solid or a liquid, the only way to increase volume
is to add more solid or liquid, so the mass must also increase.
(I don't want to get into temperature and confuse you.)
Not so with a gas. If you want to increase the volume of a jar of gas,
all you have to do is pipe it into a bigger jar. Gas always expands
to fill whatever you put it in. So if you're working with a gas, you can
increase the volume with no change in the mass.
