Asking on behalf of the class teacher who brought
students to the lab: "Why planets are spherical?
Because things roll downhill.
You could think of a cube-shaped planet as a spherical planet
with eight humongous mountains. Whenever there's any perturbation
(e.g., an earth quake) stuff from the mountains is going to come down;
only very rarely will things go up. This planet will become more
spherical as it ages.
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Update: June 2012