This simulation has two different types of gas molecules, blue and yellow. The molecules start moving in random directions, and they collide the walls of the container elastically so that the total kinetic energy of the system remains constant.
The 400 molecules (200 of each colour) will start moving as soon as the page loads.
When the simulation starts, the gas is simulated as an ideal gas and so the molecules will not interact with each other. If you untick the 'Ideal gas' control the molecules will collide with each other elastically.
You can remove the blue and / or yellow molecules by unticking the appropriate controls towards the bottom of the page.
The average number of collisions per second between the molecules and the walls of the container is shown under the simulation, this is related the the pressure exerted by the gas.
Try changing the controls and watch the effect on the number of collisions.
The molecules may occasionally stick together when 'Ideal gas' is turned off, but they should break apart again.