Why does ice float on water?
The short answer
Ice floats because water molecules spread into a more open structure when water freezes. Ice is therefore less dense than liquid water, which is unusual compared with many substances.
The molecular arrangement
In liquid water, molecules move and can stay relatively close together. When water freezes, the molecules form a regular structure with extra space. The same mass takes up more volume, so the density becomes lower.
Why floating matters
When ice stays at the surface, it makes it harder for an entire lake or pond to freeze solid. The surface ice can act as insulation, which helps aquatic life survive cold conditions.