HOW COME? No two snowflakes are alike, but they're all hexagonal

Those crazy-eight flakes may be all over this season's sweaters, scarves, hats, gloves, holiday decorations and dinnerware. But the one place you won't see them is falling from a wintry sky.

While designers may be all about the octagon, nature abhors the stop-sign flake. Except in special cases, snowflakes are hexagons, six-sided stars, plates, and columns. (Astronomer Johannes Kepler recognized this in 1611, when he wrote his essay "On the Six-Cornered Snowflake.") Snow's "sixness" goes all the way down to the molecular level.

Each flake is made up of one or many individual snow crystals. Huge flakes dropping swiftly from a winter sky may be clumps of hundreds of crystals. Close-up photos of snow crystals reveal miniature frozen landscapes, often far more elaborate than the most intricate Valentine's Day doily. Hidden in snowflakes are starbursts, fronds and flowers, layered in kaleidoscope patterns. But even the most dissimilar snow crystals have something in common - six points.

Snow crystals are hexagonal because the water molecules they are made from link together into a six-sided lattice as the temperature drops to freezing. As millions upon millions of water molecules lock into place, the lattice gets bigger, but stays hexagonal, creating a prism-shaped snow crystal.

The crystal's final shape depends mainly on the temperature (and humidity) in the cloud region where it formed. Within a cloud, snow crystals grow facets like diamonds and sprout branches like trees. The icy branches appear because the hexagon's corners jut out, accumulating freezing water molecules faster than the rest of the crystal.

While nearly all snowflakes are six-sided, there are some that at first glance seem to break the mold. One is the so-called triangular snowflake. While these snowflakes resemble icy, three-sided triangles, a closer look reveals six sides.

According to a recent study by scientists at the California Institute of Technology in Pasadena, triangular snowflakes may form when particles like dust glom onto one side. As a result, the snowflake falls at a tilt, with some sides collecting more ice than others. The result is a snowflake shaped like a stubby "triangle," with squared-off ends.

And then there are the rare 12-sided flakes. These flakes are created when ice crystals called capped columns - hexagonal columns with a six-sided plate at each end - form not straight, but with a twist. So instead of being perfectly aligned, the six-pointed ends are 30 degrees out of sync. Presto: A pinwheel snowflake with 12 icy spokes.

Think four-, five-, or eight-sided snowflakes are an improvement on nature? Check out snowflake mistakes at snowflakefail.wordpress.com. Then view the close-up, incomparable beauty of real snowflakes at snowcrystals.com.