In the case of gas particles in a container (e.g., a real bike tire, the air duster can, and the Bike Tire model below) the pressure of the system is created by the particles hitting the walls of the container.
The pressure at a given instant is calculated as the total force applied by the particles hitting the walls at a given time and the surface area is the area of the walls.
That is also why we observe the fluctuations in the "pressure over time" plot: at a given tick, slightly different number of particles may hit the walls, and the total force applied by them might be slightly different.
Now let's explore what is the difference between the fluctuations in the plot, changes in pressure, and the stability of the system:
- Run the model with 50 particles by clicking the
button, and then clicking the 
button.
- Wait until the system stabilizes (≈20 ticks).
- Add 150 more particles using the
button three times.
- Observe the changes in the system until the system stabilizes again.
