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A bicycle chain transmits pedaling force to a rear wheel. The chain often is a key component in gear changing and wears out with use.
The wear of a chain is measured by the amount it "stretches" or becomes longer. In fact, the metal does not stretch under normal usage. Chain "stretch" is caused by wear on the pivots increasing the spacing between the moving parts.
Modern bicycle chains are generally lubed "for life" and should not be soaked in solvents or degreasers. Instead, the chain should be re-lubed periodically to renew protection between moving parts.