No Christmas landscape is complete without snow. Lots of snow. And every little snowflake is unique, everyone knows that! How come, tho?
Snow is nothing else than teeny tiny ice crystals that form in the clouds and stay solid all the way down to the ground. Water crystallizes around microscopic imperfections, like dust particles floating in the clouds. Once the initial nucleus is formed, the microscopic droplets gather around it very rapidly.
Even though all snowflakes are somewhat hexagonal (due to the geometry of water molecules), each of them grows in slightly different conditions. Some had more water droplets close by, some other was in a portion of space a fraction of a degree warmer. Each and every factor counts: the final shape of the crystal is sensitive to exactly everything.
So the shape of each snowflake is random, it’s like rolling a die with infinitely many faces. You never know what will come out and all outcomes are different. If you want to put it in more physics-pompous terms, the formation of snowflakes is a stochastic process.
In the end, each snowflake is a picture of the exact conditions in which it formed. And since it’s impossible to reproduce the exact same conditions twice, each of them paints a slightly different picture.
Like pictures, snowflakes too come out better if the scene doesn’t move too much. Indeed, if conditions don’t stay relatively constant around the budding crystals, anything can happen. Most of the times, several crystals aggregate in one big snowflake, sort of a little snowball, which look a lot more like each other.
Regardless of the conditions, it’s really hard to tell them apart anyway.