Final answer:
Water's density change upon freezing helps aquatic organisms survive in winter.
Explanation:
The property of water that prevents millions of organisms living in water from perishing in winter is its density change upon freezing. When water freezes, it expands and becomes less dense, causing ice to float on liquid water. This floating ice acts as an insulating barrier, keeping the water below in a liquid state, which is crucial for the survival of aquatic life during winter.
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