According to the kinetic-molecular theory, if the particles of a gas begin to move more slowly, the temperature is decreasing.

Here's why:

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

* Slower Particles, Less Energy: When gas particles move slower, they have less kinetic energy.

* Lower Temperature: Since temperature is directly proportional to the average kinetic energy, a decrease in the speed of particles leads to a decrease in temperature.

Think of it this way: If you heat up a gas, the particles start moving faster, increasing the temperature. Conversely, if you cool down a gas, the particles slow down, decreasing the temperature.