1. A method for simulating relative motion of objects, the method comprising: receiving, at one or more computer systems that simulate motion of objects, a correspondence between a simulated object whose motion is determined from simulations of the simulated object and a reference location associated with one or more reference objects, the correspondence being established prior to a simulation of the simulated object; receiving, at the one or more computer systems, information indicating that the simulated object is being pinched by a set of surface regions associated with the one or more reference objects; ranking each of the set of surface regions according to motion of each surface region; selecting, with one or more processors associated with the one or more computer systems, a surface region of at least one of the one or more reference objects from the set of surface regions associated with the one or more reference objects whose motion matches motion of the reference location based on the correspondence between the simulated object and the reference location associated with the one or more reference objects; and constraining, with the one or more processors associated with the one or more computer systems, the simulated object to the selected surface region of the at least one of the one or more reference objects.