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Surface-tension is caused by the attraction between the liquid's molecules by various intermolecular forces. In the bulk of the liquid, each molecule is pulled equally in all directions by neighbouring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inwards by other molecules deeper inside the liquid and are not attracted as intensely by the molecules in the neighbouring medium (be it vacuum, air or another liquid). Therefore, all of the molecules at the surface are subject to an inward force of molecular attraction which is balanced only by the liquid's resistance to compression, meaning there is no net inward force. However, there is a driving force to diminish the surface area, and in this respect a liquid surface resembles a stretched elastic membrane. Thus the liquid squeezes itself together until it has the locally lowest surface area possible.
What other shape would it form? Imagine that it "chose" to form a cube; how would it know where to put the corners?
Because of it's surface tension. Just imagine if it was a square!
Why is it that if you let a bit of liquid go in a zero gravity environment it will form a perfect sphere,
So you're saying that if I let go of some sand in space, they would form into a sphere too?
Allthough in theory it is possible for a few kilos of sand in free fall to form into a sphere I think in view of the weakness of the gravitational attraction between the particles it is very unlikely than the perturbing forces will ever be low enough to let this happen
So - an oblate spheroid? Will that do?
We're talking Solar Systems here, I think - or Saturn's rings.