* Thermal expansion: As the temperature of a substance decreases, the molecules within it have less kinetic energy, causing them to vibrate and move around less. This results in a smaller average distance between the molecules, leading to a decrease in volume.

However, there are a few exceptions:

* Water: Between 0°C and 4°C, water actually expands upon cooling. This unusual behavior is due to the unique structure of water molecules, which form hydrogen bonds. These bonds become more ordered as the water cools, causing the molecules to spread out slightly.

* Phase changes: If a substance changes phase (e.g., liquid to solid), the volume change can be significant. For instance, water expands when it freezes, which is why ice floats.

In summary:

* Most substances shrink in volume when they cool.

* Water expands upon cooling between 0°C and 4°C.

* Phase changes can lead to significant volume changes.