Since it takes 5,568 years for an amount of 14 C to decay by 50 percent (half), if a specimen has one half the amount of 14 C as a modern piece of organic matter might have, we conclude it is about 5,568 years old.
Here's an analogy: Imagine you have a gallon of water to which you add one ounce of blue dye.
Archaeology and other human sciences use radiocarbon dating to prove or disprove theories.
Over the years, carbon 14 dating has also found applications in geology, hydrology, geophysics, atmospheric science, oceanography, paleoclimatology and even biomedicine.
The carbon-14 atoms are always decaying, but they are being replaced by new carbon-14 atoms at a constant rate.
At this moment, your body has a certain percentage of carbon-14 atoms in it, and all living plants and animals have the same percentage.
There's carbon-13, or 13 C, which is much rarer, accounting for only 1.11 percent, and then there's carbon-14, or 14 C, which makes up a ridiculously tiny fraction of existing carbon. Living organic matter will have steady and predictable concentrations of each isotope of carbon, pretty much the percentages mentioned above. After something dies, the 14 C decays over time (because it is radioactive) and doesn't replenish as it would in a live specimen because the dead thing isn't eating and breathing or otherwise exchanging molecules with the outside world anymore).
Cosmic rays enter the earth's atmosphere in large numbers every day.
For example, every person is hit by about half a million cosmic rays every hour.
It is not uncommon for a cosmic ray to collide with an atom in the atmosphere, creating a secondary cosmic ray in the form of an energetic neutron, and for these energetic neutrons to collide with nitrogen atoms.
When the neutron collides, a nitrogen-14 (seven protons, seven neutrons) atom turns into a carbon-14 atom (six protons, eight neutrons) and a hydrogen atom (one proton, zero neutrons).
Radiocarbon, or carbon 14, is an isotope of the element carbon that is unstable and weakly radioactive. Carbon 14 is continually being formed in the upper atmosphere by the effect of cosmic ray neutrons on nitrogen 14 atoms.