This models the "robotic" folding pen we often give as freebies at conferences and exhibitions. The "cover" is made of two shells that are actuated by arms that are attached to spring-loaded pivots, or revolute joints. The joints are also slightly damped, giving the retracting shells an almost robotic motion. This is very straightforward to model using MapleSim's multibody components. The resulting animation is pretty cool, too!