All materialworlds simulations are run by the same materialworlds simulation engine. The simulation engine then simply lets the objects interact with each other, and displays what happens. Because simulations can take a substantial slice of the computer's attention, materialworlds lets just one of them run at a time. So if there are different simulations visible on a webpage, only one will be running at any one time. To activate another, just click on it.
Simulations can carry on forever as objects continue to interact.
Without user interaction, what happens in a simulation depends only by its starting state. The main simulation view window has a configurable toolbar. The viewer can also interact directly in the 2D views of many simulations - by pulling at objects with the mouse and using keyboard commands to copy, paste or delete selected objects. The faster the better, but older computers should still work - if slowly. Materialworlds will still work with 256 or 16 colors, but some webpage colorschemes won't look right.