* Density is mass per unit volume: Density (ρ) is calculated as ρ = m/V, where m is mass and V is volume.

* Expansion increases volume: When a substance is heated, its particles gain kinetic energy and move further apart. This results in an increase in the substance's volume.

* Mass remains constant: While the volume increases, the mass of the substance stays the same.

* Lower density: Since the volume increases while the mass remains constant, the density (mass/volume) decreases.

In simpler terms: Think of it like a balloon. When you blow air into it (heat it), the balloon expands (increases volume). Even though the amount of air (mass) stays the same, the balloon becomes less dense because it's spread out over a larger space.

Exceptions:

* Water is a notable exception: Water actually becomes *less* dense when it freezes into ice. This is due to the unique structure of water molecules in solid form.

Let me know if you'd like more details or have any other questions!