Here's why:

* Temperature and Kinetic Energy: Temperature is a measure of the average kinetic energy of the molecules in a substance. Kinetic energy is the energy of motion.

* Cooling and Energy Loss: When you cool a gas, you're removing energy from the system. This means the molecules have less kinetic energy.

* Slower Movement: With less kinetic energy, the molecules move slower on average.

Think of it like this: Imagine a bunch of marbles bouncing around in a box. If you heat the box, the marbles move faster and bounce around more vigorously. If you cool the box, the marbles slow down and don't bounce as much.

The same principle applies to gas molecules.