Here's why:

* Water molecules are polar: They have a positive and negative side, allowing them to bond with each other in specific ways.

* Crystal Structure: These bonds form a hexagonal lattice structure when water freezes.

* Hexagonal Plates: The most common basic shape of an ice crystal is a flat, hexagonal plate.

While hexagonal plates are the most common, ice crystals can also take on many other shapes depending on temperature and humidity conditions. These shapes include:

* Needles: Long and thin crystals

* Columns: Short and thick crystals

* Dendrites: Branching crystals (like snowflakes)

However, the fundamental building block of all ice crystals is the hexagonal structure, which is why all of them have at least six sides.