Here's why:

* Temperature is directly related to the average kinetic energy of molecules. Kinetic energy is the energy of motion. The faster the molecules move, the higher their kinetic energy, and the higher the temperature.

* While individual molecules move at varying speeds, temperature represents the average speed. Some molecules will be moving faster, some slower, but temperature reflects the overall average motion.

Important Note: While temperature is directly related to the *average* speed of molecules, it's not the same as the speed of any single molecule. A single molecule can have a speed that is much higher or lower than the average.