It expands, which reduces its density.
It isn't unusual.
The molecular bonds cause water to expand when it freezes.
when water freezes, it expands. Most elements contract when frozen.
Water is one of the few substances that expands when it freezes. Most things contract as they get colder.
Its very unusual for a substance to expand when it freezes, water is just odd that way. Its just a property of water, its moleucles expand in the area they take up and take up more space.
It actually expands, instead of contracts, for a few degrees below the freezing point.
When it freezes.
Water is an unusual liquid in that it expands as it freezes. This can cause it to break a glass container as the water content freezes.
Water is unusual in that as it freezes it expands, thus water's density is less when a solid than as a liquid, interestingly water is most dense around 4C.
Water is unusual because it expands as it freezes and ice is less dense then water. This allows ice to float on water. This makes life on this planet possible because water freezes from the top of a lake down and not the other way around. This means that water life can survive in the winter. Also, because ice floats and doesn't sink, the land isn't flooded by the displaced water.
It floats when it freezes.
Cold water freezes faster then warm water.