* Gases: Gas molecules are constantly moving and colliding with each other. When you cool a gas, you reduce the kinetic energy of these molecules. This causes them to move slower and collide less frequently, leading to a decrease in volume, hence contraction.

* Solids: In solids, atoms or molecules are tightly packed together in a fixed arrangement. Lowering the temperature causes the atoms to vibrate less vigorously. This decrease in vibration results in a smaller average distance between the atoms, leading to a slight contraction of the solid.

Exceptions:

* Water is a notable exception to this rule. Water expands when it freezes. This is because the hydrogen bonds in water molecules arrange themselves in a crystalline structure that takes up more space than liquid water.

* Some materials have unusual thermal expansion properties, particularly at very low temperatures.

Let me know if you'd like to know more about the exceptions!