Learners compare the properties of two balls that appear identical. One ball bounces, while the other ball "thuds." The “bounce” ball is made of the polymer polybutadiene (-C4H4-). The long chains of polybutadiene are linked together with sulfur (S) atoms. This molecular structure springs back when it is compressed producing a substance that bounces. The “thud” (non-bouncy) ball is made from a combination polymer, poly(styrene-butadiene) (-C8H6-C4H4-), which has more closely packed molecules. This structure does not bounce back but absorbs the energy of the impact. Both types of polymer are used in automobile tires so that tires will “spring back” and also absorb the force of the bumps on the road.