Particle-systems and subobject-systems are a new attempt to simulate
Fire, smoke or clouds are examples for substances built of small particles which usually have a very simple geometry, say a point, whereas herds, flocks or schools of animals are built of the participating individuals, which, of course, have a much more complex geometry.
The problems that one faces trying to simulate those systems is the difficult interactive construction and the enormous computing performance which is needed to calculate the system's state for every simulation step. This attempt tries to split up the complex problem instead of, for example, solving a system of non trivial partial differential equations. This leads to the object oriented programming paradigm and efficient parallel computing.