Why can freezing water crack rocks and sidewalks?

Freezing water can crack rocks and sidewalks because of a unique property of water: it expands when it turns into ice.1

While most substances become denser as they cool, water behaves differently as it approaches its freezing point. When water cools below 4 °C (39.2 °F), its molecules begin to arrange themselves into a rigid, open hexagonal lattice. This structure creates more space between the molecules than is found in the more closely packed liquid state, causing the water to increase in volume.123

When water seeps into cracks and fissures in rocks or pavement and subsequently freezes, this expansion exerts significant mechanical force. This process, often referred to as physical weathering, acts like a wedge, gradually widening the gaps and eventually causing the material to disintegrate or crack.145