Here's why:

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

* Kinetic Energy: Kinetic energy is the energy of motion. The faster the molecules move, the higher their kinetic energy.

* Temperature and Speed: Therefore, a higher temperature means the molecules are moving faster on average, resulting in a higher average speed.

Note: While temperature is the primary measure, there are other related concepts:

* Root-mean-square speed (RMS speed): This is a specific calculation that takes into account the distribution of speeds within the gas. It's a more precise measure of the average speed, but temperature still provides a general indication.

* Average speed: This is another measure of average speed, but it's less accurate than RMS speed because it doesn't account for the distribution of speeds as well.

Let me know if you'd like more details on any of these concepts!