A Simulation, or “sim”, is a method in computer animation which resolves to a real world behavior using the known laws of physics, or a supplied system of rules developed for the purpose of testing and investigation. You are most likely familiar with dynamics, cloth and hair; each of these methods are simulation-driven. You can create cloth and hair without a simulation, if that approach suits your need (if you like the results, there is nothing wrong with it). However, if a real world result is preferred, then a simulation is the only way to get there when physical objects interact. You can even drive characters with simulations; sometimes, the desire is a “rag doll effect,” while at other times, you might wish to simulate armies. The desired result will dictate the approach that you take.