Caption: An animation of gas molecules in motion at finite temperature.

Features:

This is a cross section of a cubical box.
The number of molecules is N = 32 and say the length of a box side is L = 3. So the number density
```
  n = N/V = N/L**3 = 32/27 ≅ 1.2 . 
```
The macroscopic effect of the collisions of the molecules is pressure---which is force per unit area.
Pressure in solids and liquids is different. It originates in the resistance to compression of closely packed atoms and molecules.
Temperature, among other things, is a measure of the kinetic energy of the molecules.
The lower temperature, the lower the kinetic energy of the molecules.
The classical physics formula for kinetic energy is
```
  KE = (1/2)mv**2     .
```
The classical physics absolute zero is when all the kinetic energies of the microscopic particles are zero: i.e., all the velocities are zero.
The classical physics absolute zero (temperature) is the coldest since you CANNOT have less motion than none.
The above statement about absolute zero has to be modified for quantum mechanics.
In quantum mechanics, there is a irremovable amount of kinetic energy called the zero-point energy.
In quantum mechanics, absolute zero (i.e., the coldest) is when you reach the zero-point energy.
The Kelvin scale for temperature has its zero point at absolute zero which makes the Kelvin scale the most useful temperature scale for many formulae and calculations.