Pressure from Molecular Collisions

A gas looks mostly empty, but it's full of atoms flying around at high speed in every direction. Each one is a tiny bullet. When an atom hits a wall, it bounces off, and that bounce means the wall pushed on the atom to reverse its momentum. By Newton's third law, the atom pushed back on the wall just as hard. One collision is a flicker, gone in an instant. But billions of atoms are hitting every square centimeter of the wall every second, so the flickers blend into one steady force you can measure as pressure.

Trace a single atom: it approaches the wall with some momentum directed inward, bounces off, and leaves with the same speed but momentum directed outward. Its momentum changed, so it exerted a force on the wall during contact, by Newton's second and third laws together. Multiply that tiny force by the number of collisions per second across the whole wall, and you get pressure, force spread over area.

This is why pressure isn't the weight of gas settling onto the floor of a tank, the way sand piles up. Gas pushes equally on the sides and the ceiling of its container, not just the bottom, because molecules fly in all directions and strike every surface. That sets up the next idea: squeeze the same atoms into a smaller volume without slowing them down, and they hit the walls more often, so pressure rises in exact proportion. This is the molecular picture behind Boyle's law and the ideal gas law that follow.