in an antenatal class with ten other expectant mothers with the same due date, you can’t be sure exactly when any one of the babies will be born but you can make a pretty good estimate of the date by which 50% of them are likely to have been born. The bigger the sample, the better the estimate. For radioactivity, this time taken for half of the atoms in a sample to decay depends on the detailed specifics of the type of nucleus in question, and is called the half-life. For carbon-14, the half-life is 5,730 years, which is just right for estimating ages of objects derived from living things over
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