Here's why:

* Molecules are the building blocks: Liquids are made up of molecules. The number of molecules in a specific volume of liquid is determined by its density and chemical composition.

* Temperature affects movement: What changes with temperature is the energy and movement of the molecules.

* Cold: Molecules move slower and are closer together.

* Warm: Molecules move faster and are farther apart.

Think of it this way: Imagine a room full of people. The number of people in the room stays the same, but when the temperature rises, everyone starts moving around more, taking up more space.

Let me know if you have any other questions!