Latex (hardened)

When natural rubber is collected from a rubber tree, a milky white substance called latex bleeds from a fresh cut in the bark. The liquid latex is an emulsion containing many micro-sized particles of polyisoprene, and when the liquid dries the particles break down and the polymers bind into a continuous solid. It is the structure of these long chain polymers, and their relationship with one another, that gives rubber the ability to stretch and bend without breaking. The long chains are all intertwined and entangled, and they are quite happy to sit coiled up together due to an electrostatic attraction between them. 

When rubber is stretched, the individual polymer chains are stretched too, but the attraction between the chains is enough to keep them stuck to one another, such that when you release the rubber, all the chains pull back to their original positions. Latex liquid, as used in craft and design application, is synthetic and made from various types of polymers and elastomers.