A sphere is the 3D version of a circle: every point on its surface is the same distance (the radius) from the center. Soap bubbles, planets, marbles — when nature has the choice, it picks a sphere.
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5
volume = 4/3·π·r³ = 523.6
Volume
Surface area
Among all shapes with the same surface area, a sphere encloses the most volume. That's why bubbles round up — surface tension is minimising the surface for the air inside.