* Thermal expansion: As the temperature of a substance decreases, the molecules within it slow down. This results in less vibrational energy, and therefore, less distance between the molecules.

* Reduced kinetic energy: Colder temperatures mean the molecules have less kinetic energy, causing them to move less and occupy less space.

Exceptions:

* Water: Water is a unique exception. Between 0°C and 4°C, water actually expands as it cools. This is due to the hydrogen bonds between water molecules, which cause them to arrange themselves into a crystalline structure that takes up more space.

* Some alloys: Certain alloys also exhibit unusual behavior and may expand upon cooling.

In general, though, for the majority of substances, a decrease in temperature leads to a decrease in volume.