Original: When plants fix atmospheric carbon dioxide ( ) into organic material during photosynthesis they incorporate a quantity of 14C that approximately matches the level of this isotope in the atmosphere ( a small difference occurs because of isotope fractionation , but this is corrected after laboratory analysis ) .
Simplified: He first demonstrated the accuracy of radiocarbon dating by accurately estimating the age of wood from an ancient Egyptian royal barge of which the age was known from historical documents .