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This question already has an answer here:

As the topic suggests why the raindrop is spherical in shape? Why it is not triangular or bipyramidal or tetrahedral? Is centre of mass or density of water related to it?

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marked as duplicate by John Rennie, Qmechanic Aug 18 '15 at 18:19

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For a given volume (for raindrop - a given amount of water translates to volume with the relevant density value) - the shape with the least surface area is a sphere. This is important because there is an energetic difference between molecules inside the drop and on it's surface - molecules inside the sphere have more connections to other molecules , which decreases their potential energy (since bounded objects have negative potential energy, with respect to 0 potential energy at infinity). Thus - looking at this from the other side - the molecules on the surface have fewer connections and more potential energy. The raindrop, as every physical system, "wants" to achieve the physical state with the least potential energy (which is stable) - and thus the requirement for minimum surface area - minimal ammount of the energetic molecules for a given amount of water.

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If it was just standing with no gravity then round to minimize surface tension.

I forget the name of the law but a system will react to minimize external forces. If the drop is falling the drop takes a shape to minimize the external force of the wind resistance. It seeks an aerodynamic shape. The surface comes into play as more surface is more surface tension.

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