* Milligrams (mg) and kilograms (kg) are mass measurements, not liquid measurements. They measure how much matter an object contains.

* Milligrams are very small units of mass (1000 milligrams = 1 gram).

* Kilograms are larger units of mass (1000 grams = 1 kilogram).

* Gallons (gal) are volume measurements, specifically for liquids. They measure how much space a liquid occupies.

In summary:

* Mass: Milligrams and kilograms

* Volume (liquids): Gallons