Why do snowflakes have six sides?

Because of the shape water takes when it freezes. Water molecules lock together at an angle that naturally builds a six-sided lattice, and every arm grows outward from that.

All six arms grow at the same time, through the same air, which is why they grow so alike. And since each flake takes its own path down through different temperatures and dampness, the pattern it ends up with is a record of the journey it took.