Snowflakes show how each has a hexagonal crystalline structure with a unique geometry.
A snowflake begins when a tiny dust or pollen particle comes into contact with water vapor high in Earth's atmosphere. The water vapor coats the tiny particle and freezes into a tiny crystal of ice. This tiny crystal will be the "seed" from which a snowflake will grow.
The molecules of water that form each tiny ice crystal naturally arrange themselves into a hexagonal (six-sided) structure. The result will be a snowflake with six sides or six arms. Ice crystals are "minerals" because they are naturally occurring solids with definite chemical composition and an ordered internal structure.
The newly-formed ice crystal (snowflake) is heavier than the surrounding air and it begins falling. As it falls towards Earth through the humid air, more water vapor freezes onto the surface of the tiny crystal. This freezing process is very systematic. The water molecules of the vapor arrange themselves so that the hexagonal crystal structure of ice is repeated. The snowflake grows larger and larger as it falls, enlarging the hexagonal pattern.
Although all snowflakes have a hexagonal shape, other details of their geometry can vary. These variations are produced by different temperature and humidity conditions through which the snowflake falls. Some temperature/humidity combinations produce flakes with long needle-like arms. Other conditions produce flakes with wide flat arms. Other conditions produce thin, branching arms.
These different shapes have an unlimited number of variations, each representing the conditions of temperature and humidity and water vapor the snowflake encountered during its fall.