Physicist Kenneth Libbrecht of Caltech is fascinated by snowflakes (here in LEGO by lego_mancer). Snow crystals (flakes are technically blobs of conglomerated snow crystals) form when a tiny droplet of water freezes in a cloud at about -10 degrees C. The initial ice crystal will form a hexagonal prism due to the symmetry of how water molecules interact. After the first solid crystal forms, supercooled water vapor will go directly from the vapor to solid phase, starting at the six corners of the prism. As the crystal tumbles through the cloud, water condenses faster and slower, giving the beautiful complex shapes we've all known since we cut snowflakes out of paper in a grade school art class.
Learn much more at his site SnowCrystals.com. This site is very well written for the non-scientist, and you can learn things about the reason for the hexagonal shape, why snow looks white, whether any two crystals are actually the same, and how to grow artificial snow crystals. There are also tons of beautiful photos. Give yourself a Christmas treat and check out the site.