According to Elmhurst College, rubber can expand because the molecules inside it are coiled up like a spring. Those molecules can then be expanded or compressed, giving rubber its characteristic elasticity.
Elmhust also asserts that the process of natural rubber formation was improved with the accidental invention of vulcanization by Charles Goodyear. He found that mixing natural rubber with sulphur causes the rubber to become more durable. This is because sulphur reacts to form cross-linkages between the rubber molecules, causing the overall substance to be more resistant to stretch, compression, heat, cold and overall wear and tear. Goodyear's vulcanized rubber went on to become a cornerstone of the American automobile industry.