Here's why:

* Conservation of Mass: The fundamental principle of conservation of mass states that matter cannot be created or destroyed, only transformed.

* Phase Change: Condensation is a phase change where a gas transitions into a liquid. The molecules themselves are not changing, just their arrangement and energy levels.

* Density: The density of the substance increases significantly during condensation, as the molecules become more closely packed. However, the total mass of the substance does not change.

Think of it this way: Imagine you have a balloon filled with air (gas). If you cool the balloon, the air inside condenses into liquid water. The water droplets will take up less space in the balloon than the air did, but the total amount of matter (and therefore mass) remains the same.