Water expands as it freezes, which is why ice floats instead of sinking to the bottom.

The longer version

Freezing water pushes its molecules into an open hexagonal arrangement held by hydrogen bonds. That structure takes up more room than the liquid did, so the same mass occupies a larger volume and the density falls. Because the solid is lighter than the liquid around it, buoyancy carries it upward.

The expansion is powerful enough to crack rigid containers and break up rock, since the growing crystals wedge into tiny gaps. Most liquids shrink as they solidify and sink in their own melt, which makes water unusual. This odd behaviour is also why ice forms at the surface first.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.