Here's why:

* Temperature and Molecular Motion: Temperature is a measure of the average kinetic energy of the molecules in a substance. Kinetic energy is the energy of motion. So, the higher the temperature, the faster the molecules are moving.

* Cooling and Energy Loss: Cooling an object means removing heat energy from it. As heat energy is removed, the molecules lose kinetic energy and therefore slow down.

* States of Matter: This slowing down effect is most dramatic when a substance transitions from a gas to a liquid, and then from a liquid to a solid. In gases, molecules move freely and at high speeds. In liquids, they are closer together and move more slowly. In solids, they are tightly packed and vibrate in place.

Important Note: Even at extremely low temperatures (near absolute zero), molecules still have a tiny amount of vibrational motion. They never completely stop moving.