Ice expands as it freezes.
We’ve all seen what happens to milk after we put it in the freezer – we’ve seen how milk cartons expand.
The reason for this is simply that as water cools it traps air into its structure. The minimum volume for water is about 4 degrees, and after this it increases in volume as it takes air into it as it cools.
And this of course is why ice floats – the air it contains makes it less dense than water.