Water expands as it freezes into ice. As a result, ice takes up more room than the water it was made from. This makes ice lighter and less dense than the water below it. This allows ice to float on top of the water, rather than sink to the bottom. It is this unique property of water that allows people to walk and skate on frozen lakes while at the same time fish are able to swim below the layer of ice.
Ice can also cause problems if it forms in pipes.
Since water expands as it freezes into ice, ice can break pipes as it forms in them.