“Normal” carbon is carbon-12, an atom having 6 protons and 6 neutrons. Carbon-12 accounts for 98.93% of the Earth’s carbon. By the action of cosmic rays from space on the upper layers of Earth’s atmosphere, nitrogen atoms (7 protons, 7 neutrons) can occasionally be transformed into a weird isotope of carbon, carbon-14 (6 protons, 8 neutrons). Those weird carbon-14 atoms can then react with oxygen to form CO2, just like “normal” carbon. But carbon-14 is unstable, decaying radioactively back to nitrogen with a half-life of about 5,700 years. In the Earth’s atmosphere, the rates of creation and decay of carbon-14 are in balance, such that about 1 out of every 1,000,000,000,000 CO2 molecules in the air contains one of these weird carbon-14 atoms. But no so with carbon dioxide produced by the burning of fossil fuels. You see, the carbon in fossil fuels has been buried deep in the Earth for millions of years, with no contact with the atmosphere. All of the carbon-14 has long since decayed to nitrogen without replacement. So, CO2 produced when fossil fuels are burned contains no carbon-14.