carbon-14
"Everyone knows that there exist in nature "old women." These "grans," as they are technically referred to, live nearly forever, relentlessly refusing to give up the ghost, stretching their lifetimes out indefinitely. Carbon-14 is one of these, too. As nuclei go, it may be pretty famous, but it's famous because it's anomalous... it lives too long."
Carbon 14 is an unstable isotope of carbon. As time passes it looses some of it's mass through beta decay. It takes about 5,730 years for a sample to loose half its weight. Think of it as a wool sweater that takes a long time to shrink to 1/2 its initial size.
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