The main reason for this is that water can exist in all three phases at common temperatures found in the atmosphere.
Water is the only known substance that commonly exists in all three phases at once.
The speed of the individual water molecules determines what phase each molecule exists in. As you can see in the graphic above, the slowest moving molecules exist as ice. The water molecules in ice are locked into a hexagonal crystal and do not move freely. Liquid water molecules are moving fast enough to break free from the crystal structure, but they are still attached to each other. This allows liquid water to assume the shape of its container.