Passage: Radioactive dating relies on the decay of unstable isotopes into stable daughter products at a constant rate, known as the half-life. For example, Carbon-14 is produced in the upper atmosphere and absorbed by living organisms. When an organism dies, it stops absorbing Carbon-14, and the existing amount begins to decay into Nitrogen-14. By measuring how much of the original Carbon-14 remains relative to the amount present in living organisms, scientists can estimate the time since death. However, this method is only effective for samples up to about 50,000 years old, as the amount of Carbon-14 becomes too small to measure accurately after that point. For older geological samples, scientists use isotopes with longer half-lives, such as Uranium-238, which decays into Lead-206 over billions of years. A critical assumption in all radioactive dating is that the system has remained 'closed'—meaning no parent or daughter isotopes have been added or removed by external processes like leaching or metamorphic heating. If the system was 'open,' the calculated age will be either over- or under-estimated.