Final answer:
When particles in a substance slow down, their average kinetic energy and thermal energy decrease, leading to a lower temperature.
Step-by-step explanation:
When the particles in a substance slow down, the average kinetic energy of those particles decreases. This decrease in kinetic energy, in turn, leads to a reduction in thermal energy, which is the total kinetic energy resulting from the random motions of all the particles within a substance.
As kinetic energy is directly proportional to temperature, a decrease in the average kinetic energy causes the temperature of the substance to drop. In essence, when substances lose energy, such as through the transfer of thermal energy to another substance or the surroundings, their particles move more slowly, their average kinetic energy decreases, and their temperature decreases accordingly.