Here's why:

* Boiling Point: 100 degrees Celsius is the boiling point of water at standard atmospheric pressure. This means water will start to change from a liquid to a gas (steam) at this temperature.

* Equilibrium: At exactly 100 degrees Celsius, there's an equilibrium between liquid water and water vapor. This means some water will be evaporating into steam, and some steam will be condensing back into liquid water.

So, while technically it's still mostly liquid at 100 degrees Celsius, it's actively transitioning into a gaseous state.