ON HIVES, POMEGRANATES, AND ICE. “Kepler wondered why a hexagon is the underlying form for a snowflake and not another shape, he proposed that it may be something to do with a hexagon being one of only three shapes that can fill a 2D surface without any gaps. The others being a triangle and a square.”
http://thesmarthappyproject.com/hexagon-geometry-snowflake/