"Water is made up of one oxygen atom and two hydrogen atoms. When the water molecules bind together to form crystals, the oxygen atom shares its hydrogen atoms with neighbouring oxygen atoms and in turn borrows two other hydrogen atoms from other water molecules.
The crystal, therefore, is constructed so that each oxygen atom is connected to four hydrogen atoms.
The crystal structure that emerges has something in common with oranges being stacked at the grocer where three oranges on one layer have a fourth orange placed on top to make a tetrahedron. But look instead at each layer of oranges and you see hexagons everywhere. It's these hexagons that appear in the crystals of ice that are key to the snowflake's shape. That this molecular structure based on the hexagon translates itself to large-scale flakes with six arms is still quite surprising. It seems that as the snowflake grows, the water molecules attach themselves to the six points of the hexagon."
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