Explore how the amount of cross-linking introduced in rubber by the process of vulcanization increases its strength.  Drag the red diamonds attached to the "rubber bands" below to stretch them.  The force required to stretch them is represented by the length of the arrows, and the force vs. displacement is plotted on the graph.  Which rubber is "stronger," the vulcanized one or the unvulcanized one?  Can you relate the strength to the amount of cross-linking introduced by vulcanization?