As the U.S. east coast hunkers down for a historic winter storm, you might wonder how 50-plus centimeters of snowflakes could all be unique. A new video from the American Chemical Society explains it: All snowflakes start off looking basically the same, but they are individually shaped by their environments as they form and fall. First, water vapor condenses and freezes on a dust mote. The snowflake starts as a six-sided prism—six sides because of the way water molecules arrange themselves into ice crystals. As it grows, its edges expand and create branches of ice that shoot out from its corners. Differences in temperature and wind conditions turn the branches into one-of-a-kind works of art: Some are long and thin, some are short and fat, and some are covered in dozens of tiny sub-branches. By the time the snowflakes fall to the ground, they are so diverse that as of 2013, scientists needed 121 categories to describe them.