I was just curious... what is the practical reason behind having two separate units for measuring volume? For instance, we can use cubic centimeters and mL interchangeably in practical medicine, i.e. injections. But we tend to use cubic (centi)meters for solids, and liters for liquids/gasses. Why don't we measure all volume by the same unit? That is, why don't we measure the volume of an gold ingot in terms of liters? Or perhaps the volume of a bucket of water in cubic meters?