Here's why:

* Mass is a fundamental property of matter that represents the amount of "stuff" in an object. It's a measure of inertia and how strongly an object is attracted by gravity.

* Expansion is a change in volume, meaning the substance takes up more space. This happens when the particles within the substance move further apart.

While the volume changes during expansion, the total number of particles (and therefore the total mass) stays constant. Imagine a balloon. When you blow air into it, it expands, but the amount of air (and its mass) doesn't change.

Important Note: While mass remains constant, density changes. Density is mass per unit volume, so when a substance expands, its density decreases.