Watch The Birth Of A Snowflake

via io9.com 

The ice crystal(s) in snowflakes owe their six-fold rotational symmetry to the hydrogen bonds in water molecules. As water freezes, water molecules bound to other water molecules crystallize into a hexagonal structure, where each point on the hexagon is an oxygen atom and each side of the hexagon is a hydrogen bonded to an oxygen. As freezing continues, more water molecules are added to this microscopic six-sided structure, causing it to grow in size into the six-sided macroscopic structure that we recognize as snow flakes.

Water's tendency to expand as it nears freezing – rather than condense, like most substances – is part of what lends its frozen crystal structure its unique shape. (It's also responsible for a number of water's unique properties, and why it's so essential to the existence of life.) Something to keep in mind the next time you're experimenting with good old H2O.