Abstract: We present an effective production-proven dynamics system. It uses an explicit time differencing method that is efficient, reasonably accurate, conditionally stable, and above all simple to implement. We describe issues related to integration of physically based simulation techniques into an interactive animation system, present a high level description of the architecture of the system, report on techniques that work, and provide observations that may seem obvious, but only in retrospect. Applications include rigid and deformable body dynamics, particle dynamics, and at a basic level, hair and cloth simulation.