In a solution, particles move randomly. If any two particles collide along a horizontal axis, those particles 'stick' to each other and precipitate out of solution, collecting on the bottom of the container.
You can initialize the particles in solution by hitting the solution with the finger tool, that will then wash out the solution and replace it with a new solution of specified density.
You can change the likelihood that a collision results in a precipitation event by adjusting the stickiness