* Kinetic Energy: Heat is a form of energy. When you heat a liquid, you're increasing the kinetic energy of its molecules. This means the molecules are moving faster and with more vigor.

* Increased Vibrations: The faster movement leads to increased vibrations between the molecules. These vibrations cause the molecules to push against each other more forcefully.

* Expansion: The increased vibrations and forces overcome the attractive forces holding the molecules together. As a result, the average distance between molecules (the interparticle space) increases.

This is why liquids generally expand when heated.