The water molecules shown above are randomly placed and randomly oriented. At the root of Stat. Mech. is the Heisenberg Uncertainty Principle.
Pressure is the result of the molecules all imparting momentum to a wall.
Mech. is an easy way to treat a very complicated quantum mechanical problem of many particles. The Fundamental Postulate of Statistical Mechanics. Each particle can have a different energy - often quantized, as shown below. The particles in each system are distributed differently among the energy levels. In the laboratory, we can't measure the indiviual energy levels of each particle.