Carbon dating can only be used to find the age of things that were once alive, like wood, leather, paper and bones.
If you have a wooden box, carbon dating can tell you when the tree to make it was cut down but not when the box was made. Carbon dioxide is made into simple sugars and it is these that are the building blocks that make up wood, bark and leaves.
All food ultimately comes from green plants making sugars from carbon dioxide.
So all living things contain exactly the same proportion of carbon-14 compared to carbon-12: the proportion in the atmosphere. A living adult human body contains about a billionth of a gram of carbon-14.
We can use the same idea to find out how long it would take for a sample with radioactivity 120 Bq to drop to 30 Bq. We can use radioactive decay to calculate the age of things.
A common school experiment is to find the half-life of an isotope called protactinium-234m.
They get less radioactive in a way that's called an exponential.
Exponential decay means that equal periods of time give equal changes in radioactivity.
As it's produced carbon-14 reacts with oxygen to form carbon dioxide and some of it is taken in by green plants and made into sugars along with the 'normal' carbon.
Carbon-14 is produced all the time but it also decays all the time back into nitrogen-14.
Remember that the carbon-14 decays all the time whether the thing's alive or not.