Here's why:

* Water molecules and temperature: Water molecules are constantly moving. As temperature drops, the molecules slow down.

* Formation of hydrogen bonds: When water reaches 0°C (32°F), the molecules have slowed enough for hydrogen bonds to form between them. These bonds hold the molecules in a specific, rigid arrangement.

* Crystal structure: This rigid arrangement is what creates the characteristic hexagonal (six-sided) crystalline structure of ice.

Let me know if you'd like to learn more about the properties of water!