Here's why:

* Density is mass per unit volume.

* Solids have particles packed tightly together, resulting in high density.

* Gases have particles that are far apart, moving freely and colliding frequently. This loose arrangement leads to low density.

Exceptions:

* There are some rare exceptions where a solid might be less dense than a liquid. For example, water is less dense as a solid (ice) than as a liquid. This is due to the unique structure of water molecules in ice.