Here's why:

* Metallic Bonding: In metals, the outer valence electrons are delocalized, forming a "sea" of electrons that are shared amongst all the atoms. This creates strong attractive forces between the positively charged metal ions and the negatively charged electron sea, resulting in metallic bonding.

* Boiling: When a substance boils, the forces holding the molecules or atoms together are overcome. In the case of potassium, enough energy is supplied to break the metallic bonds between the potassium atoms, allowing them to transition from the liquid to the gaseous state.