Here's why:

* Kinetic Molecular Theory: This theory states that the temperature of a substance is directly proportional to the average kinetic energy of its molecules. Kinetic energy is the energy of motion.

* Molecular Speed: The higher the temperature, the faster the molecules move on average. This means that molecules at higher temperatures have more kinetic energy.

Important Notes:

* Average Speed: Temperature measures the *average* speed of molecules. Individual molecules can have a wide range of speeds at any given moment.

* Other Measures: While temperature is the most common way to relate to molecular speed, you can also directly measure the speed of molecules using techniques like spectroscopy.

Let me know if you have any more questions!