* Density is a measure of how much mass is packed into a given volume.

* Lower density means something is lighter for its size.

Let's compare the densities of these materials:

* Water: Density = 1 g/cm³

* Basalt: Density = 2.9 - 3.3 g/cm³

* Iron: Density = 7.87 g/cm³

* Granite: Density = 2.65 - 2.75 g/cm³

Therefore, water is the least dense of the four.

Here's how they rank from least dense to most dense:

1. Water

2. Granite

3. Basalt

4. Iron