Almost every substance on Earth follows the same rule: as it cools, it contracts and gets denser, and its solid form sinks in its own liquid. Water breaks that rule — ice floats on top of water.
That property may be the reason aquatic life survives winter at all:
A backward freeze. Most liquids keep getting denser all the way down to their freezing point. Water does too — it gets denser until it hits about 4°C (39°F). Below that, it reverses course and actually begins to expand as it approaches freezing, instead of continuing to contract.
Ice takes up more room. When water freezes solid, its molecules lock into an open, hexagonal lattice that takes up about 9% more space than the same water took up as a liquid. More volume for the same mass means lower density.
So ice floats. Because ice is roughly 8–9% less dense than liquid water, it floats instead of sinking — unlike nearly every other frozen substance on Earth.
A floating insulator. That floating layer of ice acts like a lid, insulating the water underneath from freezing air above. Meanwhile, the densest water (that 4°C sweet spot) sinks to the bottom, where it stays.
Life goes on underneath. Fish, frogs, and other aquatic life survive winter in that insulated layer of liquid water below the ice, even while the surface is frozen solid and the air above is far colder.
If water behaved like almost everything else, ice would sink as it formed, lakes and rivers would freeze from the bottom up, and most bodies of water in cold climates would turn into solid blocks of ice every winter — wiping out aquatic life in the process. Instead, one exception to an otherwise universal rule keeps entire ecosystems alive.
