Mechanism simulation is an essential tool for robotics and engineering. Computer programs for simulating rigid-body dynamics are usually large and elaborate pieces of software. The complexity of these systems has led to an apparently common belief that rigid-body dynamics simulators must necessarily be large and complex, and that developing such simulators will require an expert team of programmers with Ph.D.’s in Lagrangian dynamics and differential geometry. This is wrong. We claim that the mathematics and physics knowledge of even a first-year engineering student suffices in order to implement a useful rigid-body dynamics simulator, powerful enough to handle kinematical loops. Of course, the student’s simulator is unable to compete with the professional systems when it comes to features and efficiency, but it will be perfectly useful for many non-trivial applications, and most importantly, the student will be able to completely master its theory and implementation.