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For most living beings, breathing is an automatic mechanism and does not require one to think much about it.  However, for whales and dolphins, this does not apply.  They breathe via an opening situated at the top of their heads, called blowhole through which water should never pass.  These animals hence must each time come to the surface to breathe.

This is the reason why the cetaceans cannot allow themselves to sleep.  They rest naturally though part of their brain remains forever active.  Often, they even keep an eye open!

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