Why Does Rubber Bounce Back?

Step 1 of 5

It starts as tree sap.

Cut the bark of a rubber tree and a white liquid runs out. It is the tree's own defence against damage, collected in a cup, and it is where natural rubber begins.

Step 2 of 5

The molecules are tangled springs.

Rubber is made of extremely long molecules lying in a random tangle. Pull, and they straighten out. Let go, and they scramble back into a tangle, because tangled is simply far more likely.

Step 3 of 5

Raw rubber is nearly useless.

On its own it goes sticky in summer and hard and brittle in winter. Early rubber goods melted on shop shelves, and the whole trade very nearly collapsed.

Step 4 of 5

Cooking it with sulphur fixed that.

Heat rubber with sulphur and small bridges form between the long molecules. Now they can still uncoil, but they cannot slide past each other — so it stays springy hot or cold.

Step 5 of 5

Stretching it warms it up.

Put a rubber band to your lip and stretch it quickly — it feels warmer. Let it snap back and it cools. Almost no other material does anything so obvious with a simple pull.

Most rubber today is made from oil rather than trees, but tyres for heavy vehicles still use natural rubber, because nothing artificial handles the heat as well.