Answer: hydrogen bonds.
Explanation:
Liquid water molecules form a strong intermolecular attraction (force) named hydrogen bonds.
When water boils, the intermolecular forces have to be defeated to pass from a more ordered and tight arrange of molecules in the liquid state to a more disordered and loose arrange of atoms in the gas state (the molecules move freely occuping all the space).
In liquid water the molecules are packed closely together.
As the liquid water is heated, the temperature increase causes kinetic energy increases (increasing the motion and vibration of molecules).
When the molecular motion is as intense as disrupt the hydrogen bonds between the molecules, the molecules become gas (steam).
