It is also standard to coat fossils during their extraction and transport.

It is produced in the upper atmosphere by radiation from the sun.

(Specifically, neutrons hit nitrogen-14 atoms and transmute them to carbon.) Land plants, such as trees, get their carbon from carbon dioxide in the air. The same is true of any creature that gets its carbon by eating such plants. Suppose such a creature dies, and the body is preserved.

The C14 will undergo radioactive decay, and after 5730 years, half of it will be gone. So, if we find such a body, the amount of C14 in it will tell us how long ago it was alive. The method doesn't work on things which didn't get their carbon from the air.

This leaves out aquatic creatures, since their carbon might (for example) come from dissolved carbonate rock.

As the name suggests, fossil fuel is old, and no longer contains C14.

Both of these man-made changes are a nuisance to carbon dating.

If you hear of a living tree being dated as a thousand years old, that is not necessarily an example of an incorrect dating. Wood taken from the innermost ring really is as old as the tree. We can date things for which historians know a "right answer".

That causes a dating problem with any animal that eats seafood. After about ten half-lives, there's very little C14 left.

So, anything more than about 50,000 years old probably can't be dated at all.