This model simulates the behavior of gas molecules in a box with a moveable piston. The piston has weights on it which push it down, and the gas particles push upward against the piston. The system is called "adiabatic" because no heat energy (such as heat loss through the walls of the box) is added to or removed from the system.
For detailed instructions on using the program, take a look at the info window from the Adiabatic Piston model.
Click on one of the pictures to see a quicktime movie of the model: