Here's why:
* Hydrogen Bonding: Water molecules form strong hydrogen bonds with each other. In the liquid state, these bonds are constantly breaking and reforming, allowing for a more compact arrangement of molecules.
* Ice Crystal Structure: When water freezes, the hydrogen bonds lock the molecules into a very open, crystalline structure. This structure creates large spaces between the molecules, making ice less dense than liquid water.
This unusual property is what allows ice to float on water.
