* Density is mass per unit volume.

* Gases have molecules that are far apart. This means they have a lot of empty space between them.

* Liquids have molecules that are much closer together. This means they have less empty space.

Because liquids have more molecules packed into the same volume, they are denser than gases.

Think of it this way: Imagine a room full of people. If everyone is standing close together, it's more crowded (denser) than if everyone is spread out.