Water is the only common liquid that doesn't immediately contract when cooled. Ice melts, and water becomes denser, but only up to 4 degrees Celsius. Beyond that, with further cooling, it expands again. This quirk explains why fish survive in freezing ponds: the densest water sinks to the bottom, while a lighter cold layer stays on top and turns into ice.
Scientists built a neural network that replaced complex calculations and allowed them to simulate the behavior of millions of water molecules. The model accurately reproduced the temperature anomaly of density. Peering inside, researchers saw that each water molecule still holds onto four neighbors, as in an ideal ice crystal. But this order breaks down already at a distance of a few molecular layers.
This breakdown in long-range order is what makes water anomalous. When heated from zero, molecules first fill the voids created by the broken long-range order, and water contracts. After 4 degrees, ordinary thermal expansion takes over. The mechanism turned out to be subtler than thought for nearly a hundred years.
If water didn't expand when freezing, ice would sink, and lakes would freeze solid, killing everything alive.