If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis that all things are made of atoms — little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied.
~ Richard Feynman
Do you agree with Feynman? You have to. The kinetic theory of matter (or the atomic hypothesis, as Feynman called it) is able to explain so many things around us. For a start, it can explain the different states of matter.
Something is solid when it has a fixed volume and shape. The intermolecular forces among the molecules in a solid must be very strong, such that every molecule is bound by its neighbouring molecules to a fixed position, resulting in a rigid structure called a lattice.
A liquid has a fixed volume but no fixed shape. This suggests that the intermolecular forces among liquid molecules is strong enough to hold one another together collectively, but weak enough to allow for individual molecules to glide from one neighbouring molecule to another.
A gas has neither fixed volume nor shape. The intermolecular forces are so weak that the molecules move freely and independently from one another. They zip around happily, confined only by the walls of their containers.
We are just getting started. As we shall see, the kinetic theory forms the basis for all thermal concepts, including temperature, heat and internal energy.