Here's why:

* Heat energy: Boiling water involves adding heat energy. This energy causes the water molecules to vibrate faster and move more rapidly.

* Breaking bonds: The increased energy overcomes the attractive forces (hydrogen bonds) holding the water molecules together in the liquid state.

* Phase change: As the molecules move farther apart, they enter the gaseous phase, which is why you see steam.

In short, boiling water causes the molecules to move more vigorously, breaking the bonds that hold them close together in the liquid state, and leading to a significant increase in the distance between them.