Why Do Snowflakes Have 6 Arms?

Count the arms on any snowflake and you get six. The next one has six as well, even though its pattern is completely different. Every single one. So why six? Does somebody cut them out? Nobody cuts them. Ice builds itself that way — and you can see the rule at home, using ordinary salt. You will need: dark paper or a dark plate, a salt shaker, a pinch of sugar and a ×10 magnifier. Put a pinch of salt on dark paper and look through a magnifier. Every grain is a cube. Not one lucky grain — all of them. Nobody cut that shape; it came out by itself. Now put sugar next to it, same spoon, same hand — and the grains are not cubes but slanted little bars. So the shape is set by the stuff itself, not by whoever poured it: every substance has its own pattern, and the pattern repeats in every single grain. For water that pattern is a ring of six bits. Ring after ring after ring makes a net with six corners everywhere. The corners stick out the farthest, so they collect the most. That is why a bit of ice grows in six directions: six corners — six arms. Five never happens. Seven never happens. And twelve is simply two stars grown together. We left almost two seconds of silence for your answer: what pattern does ice have? Then why is every snowflake different, if they all have the same number of arms? Because each one takes its own road. A snowflake falls for half an hour and grows the whole way: where the air is damper, the arms grow fluffy little branches; where it is drier, they come out smooth and plain. And the six arms of one snowflake look alike because they grow side by side, in the same bit of cloud — the same weather for all six. And once more the line from the last episode: if a drop freezes you get a round pellet, not a star. A star never froze — it grew from vapour. Try it yourself: stir a spoon of salt into warm water, drip a few drops onto a dark plate and leave it somewhere warm for a day. The water leaves — the new cubes stay, bigger than the ones in the shaker. Not dry yet? Just wait a bit longer. A bonus for parents that is not in the film: in grown-up words this is the crystal structure of matter. Table salt has a cubic lattice, sugar a monoclinic one, and ice Ih a hexagonal one — which is exactly where the six-fold symmetry of a snowflake comes from. Each water molecule holds four neighbours at almost tetrahedral angles — it is that hydrogen-bond geometry that builds the six-membered rings (the H–O–H angle itself is 104.5°). Branching is driven mainly by supersaturation (humidity), while the crystal type is set by temperature — the classic Nakaya diagram. The words "molecule" and "lattice" are never spoken in the episode: for ages 7–10, "bits" is enough. Safety: • never point the magnifier at the sun — it burns your eye • when you are done, put the magnifier away, out of the sun • we do not eat salt or sugar by the handful, and no rubbing eyes with salty fingers • ask a grown-up before you take the salt; a grown-up pours the hot water An educational physics cartoon for children aged 7–10, with Droplet and Skimmy. #physicsforkids #scienceforkids #snowflakes #whysix #cartoonsforkids #homeexperiments #littlewhys In this episode: 0:00 We counted the arms — six again 0:28 Experiment: salt under a magnifier 0:50 Now sugar — same spoon 1:02 What pattern does ice have? 1:42 So why is every snowflake different 2:13 Why the six arms look alike 2:25 And if a drop freezes? 2:39 Safety 2:51 Try it yourself: grow your own cubes If you could not see the cubes: use DARK paper (grains are invisible on white) and a ×10 magnifier rather than ×5. Shaker salt is fine-grained and often chipped — the cubes you grow from a solution come out bigger and neater. Where snowflakes come from — the previous episode:    • Where Do Snowflakes Come From?   Next time we find out why snow is white if ice is clear. All episodes in English (playlist) —    • Science for Kids — Cartoons in English