How is rubber made stretchy?

Rubber stretches because it is built from long polymer molecules that can straighten when pulled and curl back when released. Tiny links between the strands keep the material from drifting apart, so the whole tangle returns toward its earlier shape.

That means a rubber band is not simply bending like soft wire. Inside it, countless chains are being tugged longer, then relaxing again. The named idea to remember is polymers: long molecules whose shape helps explain balloons, balls, tires, and bendy bands.