An elastomer is a polymer with few crosslinks between the polymer chains. The crosslinks allow the polymers to return to their original position after been stretched out of alignment. Since crosslinks(covalent bonds) between the polymers exist the elastomer is heat resistant. The more crosslinks the less flexible the elastomer.

Consider the demonstration shown on the left. The elasticity of the elastomer is derived from the ability of the long polymer chains to realign themselves to distribute an applied stress. Covalent crosslinks between the polymer chains ensure that the polymers return to their original configuration when the stress is removed.