Light
it depends on how much things matter to you
it depends on the two different things
All matter has mass and volume.
It depends on the context. One would be the mass.
All things should fall into the following two categories: Matter and not-matter. Anything physical (eg. a rock) is made of matter, anything spiritual, relationship orientated or arsey science suggestion is not-matter.
These is a rule of Physics, "matter can neither be created or destroyed".
heat and pressure
H2O and CO2
It all depends on the density of the matter that is in the gallon.
All matter 1) has mass and 2) occupies space (has volume).
The volume of gas depends on two things: pressure and temperature.
Depends which religion