How are particles of matter constantly moving?
Imagine a state such that no particles move at all. It is clear that this is the lowest possible energy state, as there is zero kinetic energy of the random motion. This is the picture that you would have (at least in a classical theory) at the temperature of absolute zero. Unfortunately, the rules of nature prevent a system from reaching K. You can, nevertheless, get arbitrarily close to it. Imagine now that you have this as-close-to-zero-as-possible system, where all the particles are nearly static. You add some energy to the system by bumping into one of the particles, for example. This particle has now some kinetic energy until it hits some other nearby particle, transferring part of its energy in the collision. Now both particles move around and will collide again and again with more and more particles, diluting their energy in the system. Since ordinary ensembles of particles consist of billions and billions of them, the ensuing motion is, for all intents and purposes, random. So you have some random motion of all the particles, which is (a bit incorrectly) linked to temperature. But the link is there, and the idea is that if you add any energy to a system of particles, after some equilibration time the energy gets converted into the kinetic energy of this random motion (= heat), and the system rises in temperature. So, hopefully, you see how adding energy to a system will cause it to develop this constant random motion. Even if you try to very carefully control the motion of the particles, the laws of nature, this time the second law of thermodynamics, will work against you, and sooner or later any orderly motion will devolve into the more disorderly random motion. Finally, you might wonder why the particles dont simply stop. If you imagine them as billiard balls suspended in empty space, you cant expect any friction to eat away at the individual kinetic energy of each particle. The only way how to get rid of their energy is to transfer it in a collision to another particle. But since there are random collisions constantly all around the place, any particle that manages to lose some energy in one collision will sooner or later regain it in another. And since the energy of the overall system is conserved (in fact, this can be seen as the definition of energy, as a quantity that is conserved), once the particles start going, they wont stop. There is no way for them to shed their kinetic energy, they just keep exchanging it among each other. It is only when you provide a system of a lower temperature that some of the heat of the original system can flow away, cooling the system and slowing down the random motion. But as you can not cool it down all the way to , there will always be some Brownian motion.