Algodoo

This simulation...
... aims to indicate pressure by spawning a semi-transparent circle at every collision occuring. The size of the circle depends on the momentum of the colliding particles.
... can be interacted with via
a) clicking on the areas labled as "Hot" or "Cold" to change them into their opposite (speeding up/slowing down particles they collide with)
b) deleting particles (preferably on the outer room)

Observe how your actions...
... change the visual representation of pressure
... change the movement of the pink pressure piston

General knowledge:
Matter consists of tiny particles (atoms or molecules). In gases, these particles move randomly and very fast. The particles constantly collide with the walls of a container. Each collision exerts a small force on the wall.

The total force per unit area of all these collisions is called pressure.

Higher temperature → particles move faster → pressure increases.
More particles in the same space → more collisions → pressure increases.