Here's why:

* Mass is a fundamental property of matter. It represents the amount of matter in an object. Whether a substance is solid, liquid, gas, or plasma, the amount of matter it contains (its mass) remains constant.

* Phase changes don't alter mass. When matter changes phases (e.g., ice melting to water), the molecules themselves don't disappear or change their fundamental composition. They simply rearrange themselves and move with different levels of energy.

Important Note: While the mass remains constant, the density of a substance can change with its phase. For example, water is denser as a solid (ice) than as a liquid.