Water expands when it freezes because the molecules arrange themselves into a more open, hexagonal crystal structure. This structure takes up more space than the liquid water, causing it to expand by about 9%.

When water freezes in a confined space, such as a crack in a rock, the expansion can create enough pressure to break the rock apart. This process is called frost jacking and is one of the main ways that rocks are weathered in cold climates.

Frost jacking is a serious problem for infrastructure in cold climates, as it can damage roads, bridges, and buildings. To prevent frost jacking, engineers design structures so that water cannot seep into cracks and freeze. They also use materials that are resistant to the effects of frost jacking, such as concrete with air bubbles added to it.