Here's why:

* Hydrogen bonding: Water molecules form strong hydrogen bonds with each other. In liquid water, these bonds are constantly breaking and reforming, allowing the molecules to be closer together.

* Crystal structure: In ice, the hydrogen bonds form a rigid, open crystalline structure. This structure holds the molecules farther apart than in liquid water.

The lower density of ice compared to liquid water has important implications for life on Earth:

* Ice floats: This allows aquatic life to survive beneath frozen lakes and oceans.

* Climate regulation: The expansion of water upon freezing helps moderate Earth's climate by reflecting sunlight and distributing heat.