Here's why:

* Polarity: Water molecules have a bent shape, with the oxygen atom being more electronegative than the hydrogen atoms. This creates a partial negative charge on the oxygen side and a partial positive charge on the hydrogen side, making the molecule polar.

* Hydrogen Bonding: The partial positive charge on the hydrogen atoms of one water molecule can form a weak attraction with the partial negative charge on the oxygen atom of another water molecule. These attractions are called hydrogen bonds.

These two properties, polarity and hydrogen bonding, are responsible for many of water's unique properties, including:

* High boiling point: Hydrogen bonds hold water molecules together, requiring a lot of energy to break them and allow water to boil.

* High specific heat: Water can absorb a lot of heat energy without changing temperature significantly due to the energy required to break hydrogen bonds.

* Excellent solvent: The polar nature of water allows it to dissolve many ionic and polar substances.

* Surface tension: Hydrogen bonding creates a strong cohesive force between water molecules, resulting in surface tension.

* Expansion upon freezing: The hydrogen bonding in ice creates a more open structure than in liquid water, making ice less dense and allowing it to float.

So, the combination of polarity and hydrogen bonding is the key to understanding the many unique properties of water.