Rubber bands can shrink back after being stretched, which is the function of rubber molecules. Rubber molecules love to move, they are arranged hand in hand, because each molecule is always in active motion, so their team is always crooked and messy. If you pull the rubber band hard, the rubber molecules will lose the "freedom" to move arbitrarily, and the team they are arranged in will be neat and tidy, and the appearance will be stretched. However, these rubber molecules are not willing to be lonely, they want to restore their "freedom", so they will generate a force to restore the original state, which is the elasticity of rubber. This kind of rubber is actually a kind of raw rubber.
If it is stretched to a certain extent, the molecules will "slip" and can no longer return to their original state. In order to prevent the "slip" phenomenon between rubber molecules, the rubber molecules must be connected to each other to form a three-dimensional network. This method is called cross-linking, which is as elastic as a mesh door made of iron bars. In 1839, Goodyear discovered vulcanized rubber, which uses sulfur as a cross-linking agent to make the rubber molecules in the raw rubber hold hands at several places, turning it into a high-performance, elastic rubber.
Today's rubber bands are made of this elastic rubber.
