Rubber bands, or elastic bands, are used to fasten things together by stretching to surround a bundle of objects and then retracting to hold the bundle in place. Over time, they wear out.
Rubber bands are manufactured from a tube of stretchy polymers such as rubber or latex of varying color, thickness and diameter. The tubes are cut into circles, thus producing the bands. It returns to its original shape after being stretched.
Rubber is a polymer formed from chains of molecules. It is flexible because there are not many cross links between the chains to prevent them from moving. Ultraviolet light from the sun and heat breaks up the polymer chains, causing the rubber bands to crack. Stretching them too far breaks the polymer chains.
You can make your rubber bands last longer by keeping them in a cool place out of the light, such as in a refrigerator or basement. Stretch them only as needed; overuse will make them brittle. If you really want to preserve your elastic bands, coat them lightly with castor oil.