I agree with Kashif, circular hone ka yahee faida hota hai k andar nahi girtey, kissi bhee doosri shape k andar gir sakte hain( ya phir iss cheez se bachne k lie dhakkan hole k size se ziada hee barey bnaney pren ge) round size ka thora sa bara ho hole se to no, problem.
and they say that circle is the most stable and strong shape.

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"The only necessary thing for the triumph of evil is for good men to do nothing."
--Edmund Burke (1729-1797)