Water is one of the few substances that expands when it freezes.

Most liquids contract and become denser as they cool into a solid. Water does the opposite: as it freezes, its molecules lock into a hexagonal crystal lattice held apart by hydrogen bonds, an open structure that takes up roughly 9% more space than liquid water. That’s why ice floats instead of sinking.

This quirk has outsized consequences. Because ice floats, lakes and oceans freeze from the top down rather than the bottom up, insulating the water underneath and letting aquatic life survive the winter. It’s also why water pipes burst in freezing weather — the expanding ice exerts enough pressure to crack metal and stone.