According to the Kinetic Theory of Gases, gas pressure is a macroscopic manifestation of microscopic collisions.

1. Momentum Change: When a gas molecule of mass $m$ and velocity $v_x$ hits a wall and rebounds elastically, its momentum changes by $\Delta p=m(-v_x)-m(v_x)=-2m{v}_x$.
2. Impulse: The wall exerts a force to cause this momentum change. By Newton's Third Law, the molecule exerts an equal and opposite force on the wall.
3. Averaging: The constant bombardment by billions of molecules creates a steady average force.
4. Pressure: Pressure is defined as this average force divided by the area of the wall ($P=F/A$).