Here's why:

* Water is an exception to the general rule: Most substances are denser in their solid state than in their liquid state. This is because the molecules are packed more tightly together in a solid.

* Ice floats: Water, however, is less dense in its solid form (ice) than in its liquid form. This is due to the unique hydrogen bonding structure of water molecules. When water freezes, the hydrogen bonds create a crystal lattice that forces the molecules further apart, making ice less dense.

Let me know if you have any other questions!