The volume of water at zero degrees Celsius initially decreases then increases as the temperature increases. Water is one of the few substances that this property. Most substance expands as they are heated, but water is different. As water is heated from zero degrees Celsius, its molecules slow down and become closer together, causing a decrease in volume. This process continues until the water reaches its maximum density at around four degrees Celsius. Once it passes four degrees Celsius, the water molecules start to move more quickly and spread out, causing an expansion in volume.