Why does ice float instead of sinking?

Ice floats because it is lighter, volume for volume, than the water it came from. When water freezes, its molecules lock into a hexagonal crystal held together by hydrogen bonds, and that crystal packs them less tightly than the jostling liquid does. The result: ice is about 8.3% less dense than liquid water, which means the same water takes up about 9% more room once frozen.

That sounds like a detail, but it is strange. For most other substances, the solid is denser than the liquid and sinks to the bottom. Water does the opposite, and life depends on it. Because ice floats, ponds and lakes freeze from the top down, and the lid of ice shelters the plants and animals living underneath. If ice sank, lakes would freeze from the bottom up and everything in them would die.

You have probably met the same expansion in your kitchen or your cellar. Freezing water pushes hard enough to burst water pipes, which is a common cause of flooded houses in winter. And at sea, only about a tenth of an iceberg shows above the surface, and the rest hides underwater. In 1912, the Titanic sank after striking one.