Final answer:
As a cup of coffee cools, the particles within it move slower due to a decrease in kinetic energy, which is directly related to the temperature.
Step-by-step explanation:
The particles in a cup of coffee move slower as it cools down. The kinetic energy of particles is directly related to temperature. The higher the temperature, the more energy the particles have and the faster they move. As the coffee loses heat to its surroundings, the energy of its particles decreases, causing them to slow down. This is in accordance with the principles of thermal dynamics and is reflected in the scenario where molecules in hot water move more rapidly than those in cold water.