Radiocarbon dating stuff works
And after 11,460 years (two half-lives), only a quarter of the original carbon-14 atoms are left.
After five half-lives (28,650 years) only 1/32 remains.
Radiocarbon dating is used to work out the age of things that died up to 50,000 years ago. As far as working out the age of long-dead things goes, carbon has got a few things going for it. The proteins, carbohydrates and fats that make up much of our tissues are all based on carbon.
Everything from the fibres in the Shroud of Turin to Otzi the Iceman has had their birthday determined the carbon-14 way. There's plenty of hydrogen, oxygen and nitrogen in living things too, but carbon's got something none of them do — a radioactive isotope that can take thousands of years to decay.
(You can read up on radioactivity and isotopes here).
And plants top up their radioactive carbon every time they turn carbon dioxide to food during photosynthesis.
From the moment we die the proportion of carbon-14 compared to non-radioactive carbon-12 in what's left of our bodies starts to drop as it gradually turns to nitrogen.
And the longer dead things lie around, the lower the carbon-14 levels get.
So calculating the age range of a once-living sample involves measuring the 14C/12C ratio, and using this the known half-life to estimate the length of time since the sample died.
That age range is then compared with known 14C/12C ratios from the tree ring/marine record to find the best match, and the result is a calibrated age range you can be 95 per cent sure of.