Anonymous wrote: I just do not get it....
The thing is milligrams measure mass (weight), milliliters measure volume. Conversion from volume to weight differs depending on the substance. For water (and most water-based solutions) 1 ml weighs approximately 1 mg. However 1 ml of mercury of plum is much heavier than 1 mg. On the other hand, 1 ml of oil is lighter than 1 mg.