Why do snowflakes have six sides?
The short answer
Water molecules arrange themselves in a hexagonal pattern when they freeze, giving snow crystals six-fold symmetry.
How it works
Snow crystals grow from water vapor in clouds. Temperature and humidity affect each edge as the crystal grows. The six directions tend to develop similarly, while changing conditions create different patterns.
A familiar example
With a magnifying glass, you can often see six main arms branching into smaller patterns.