This is a model of how sand particles interact with each other. In this environment all sand particles on the screen try to move down if any of the "rules" apply. The rules are conditions under which the particle may move down and which way it will move. If none of these rules apply the particle does nothing.
For detailed instructions on using the program, take a look at the info window from the Sand model.
Click on one of the pictures to see a quicktime movie of the model: