You are going to create a particular simulator.
You will be able to put sand, rocks, and a particle called floater on a simulator where sand will fall to the bottom, floater will rise and rocks will stay put.
This will all take place in a 100X100 grid of Particles (a class included). Each element will be null or have a Particle.
In the example shown, most spots are empty except some Sand (S) and a Rock (R)
Behind the scenes, move is being called on every particle. Sand will move down (if possible) each time. Rocks stay put (floater, goes up).
To do this, a sand move method would take in the grid and:
Take a look at Google Doc Notes for more.