* Thermal Expansion: As a liquid is heated, its molecules gain kinetic energy and move faster. This increased motion causes the molecules to spread further apart, increasing the volume of the liquid.

* Density and Volume: Density is defined as mass per unit volume (Density = Mass/Volume). Since the mass of the liquid remains relatively constant while the volume increases due to heating, the density decreases.

Think of it like this: Imagine a glass of water. When you heat it up, the water expands, taking up more space in the glass. Even though the amount of water (the mass) stays the same, it's now spread out over a larger volume, making it less dense.