The property of water that explains why it expands when it freezes is known as hydrogen bonding. When water cools and approaches its freezing point, the molecules lose kinetic energy and form stable hydrogen bonds, creating a crystalline structure. This structure is less dense than liquid water, causing ice to expand. This unique property is significant because it allows ice to float on water, insulating and protecting aquatic life in colder climates. This characteristic of water is crucial for environmental stability and has implications for climate and ecosystems.