Here's why:

* Molecular Motion: Heat causes the molecules within a liquid to move faster and vibrate more. This increased motion leads to greater spacing between the molecules, resulting in an overall expansion of the liquid's volume.

* Thermal Expansion: This phenomenon of liquids changing volume in response to temperature changes is called thermal expansion.

* Exceptions: While most liquids expand when heated, there are a few exceptions, such as water between 0°C and 4°C, where it behaves anomalously and contracts slightly upon heating.

Let me know if you'd like more information about thermal expansion!