What makes material bouncy?

First you need a long molecule resembling a chain. Chemists call that a polymer. Then you need a sulfur atom to form a bond from one link to another, or from one chain to another. The sulfur bond allows the chains to flop around. Rubber originally was so hard they made combs with it, but Charles Goodyear invented a way to add the sulfur bonds cheaply so the rubber would be bouncy enough to make tires.