THE AMERICAN HERITAGE® DICTIONARY OF THE ENGLISH LANGUAGE, FIFTH EDITION by the Editors of the American Heritage Dictionaries.Copyright © 2016, 2011 by Houghton Mifflin Harcourt Publishing Company.Because carbon-14 decays at this constant rate, an estimate of the date at which an organism died can be made by measuring the amount of its residual radiocarbon. The method is widely used by Pleistocene geologists, anthropologists, archaeologists, and investigators in related fields.
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A technique for measuring the age of organic remains based on the rate of decay of carbon 14.
You will notice that after around 40,000 years (or 8 half-lives), the amount left is starting to become very small, less than 1%.
Scientists often use the value of 10 half-lives to indicate when a radioactive isotope will be gone, or rather, when a very negligible amount is still left.
By measuring the amount of carbon-14 left in the organism, it's possible to work out how old it is.