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Old 10-02-2012, 08:01 PM   #28
diggler
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Join Date: Sep 2005
Location: Sydney
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I think it depends on how the balls are made.

I read somewhere that when some balls are made, a pellet is put inside when they join the 2 halves together. The pellet sublimates from solid to gas and hence creates gas pressure.

Other balls are just air filled.
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