Why do balls go up some times and sometimes they hang low?

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Why do balls go up some times and sometimes they hang low? What are physical explanations to it? Whats happening with them? I am talking about guy's balls. Someone told me that , In general, they do this to regulate temperature. The job of producing genetically 'perfect' sperm is a very important one in terms of evolution, and this is best done at a constant temperature (not too hot or too cold), so they will move closer to the body when too cold, and farther away when too warm. It is just like what you would consciously do with your hands on a cold day - tuck them into your pockets, or armpits. So it there more to it ?