Here's why:

* Temperature is a measure of the average kinetic energy of the particles in a substance. Kinetic energy is the energy of motion.

* Higher temperature means the particles are moving faster on average, and thus have more kinetic energy.

* Lower temperature means the particles are moving slower on average, and thus have less kinetic energy.

It's important to note that temperature doesn't measure the speed of individual particles, but rather the *average* speed of all the particles in a substance.