What you Need
What to do
Hold the rubber band firmly at each end.
Touch it to your lips, and remember its temperature.
Move it away slightly and stretch it as far as it will comfortably go.
Touch it to your lips again.
Hold it stretched for a few seconds, then allow it to shrink back to its normal size again.
Feel the temperature again.
Is there any difference?
What may happen
You should find that the rubber band gets slightly warmer when you stretch it, and then slightly cooler as it shrinks again.
Why does it happen?
Rubber contains millions of randomly coiled long molecules called polymers, which are each made of shorter molecules known as monomers. At certain points the long polymer chains are joined together or crosslinked, but they are otherwise free to move around or vibrate owing to the heat energy in the structure.
When you stretch a piece of rubber the coiled molecules are pulled straight; when you let go again they move around at random through thermal vibrations, which restore them to their more compact coiled configuration.
So why does an elastic band become hotter when you stretch it and colder when it shrinks?
When you stretch the rubber band you are adding energy to the rubber, which causes it to vibrate more strongly and this makes it hotter. Then, when you let go, the rubber molecules must do work to pull the polymer chains back into their compact configuration; since this process uses energy, the eleastic band must have less energy after it has shrunk and therefore it becomes colder.
Below is a simple computer model of a small rubber molecule. You can drag the green end around and see what happens to the average energy and therefore the temperature of the molecule.
Part of the show Clothed Questions - Naked Answers from the 4th May 2008