To understand the physical properties of a gas from microscopics, we employ the following idealized yet accurate kinetic description that contains the following elements:

• Gas molecules (or atoms) are structureless.
• Molecules move freely between collisions.
• Collisions occur occasionally (relatively).
• Collisions are nearly instantaneous and elastic.
• Molecular velocities are randomized upon each collision.
• Molecular speeds are distributed over a moderate range.
• The typical speed is set by the temperature.