Carbon is an element which is found in the molecules of all living things. But elements can come in various forms, called 'isotopes'. Isotopes have extra neutrons in their nuclei, and are often unstable (radioactive). The element carbon has an isotope called carbon14. Carbon14 is formed in the upper atmosphere when cosmic rays strike, producing neutrons. These then interact with nitrogen to form carbon14. Carbon14, although there isn't very much of it, mixes with ordinary carbon, and finds its way into living things when they absorb or eat their food. The ratio of carbon14 atoms to ordinary carbon atoms is constant. As long as the plant is alive, it continues to take in carbon of both types. Black dots represent carbon, grey dots carbon14 Because carbon14 is radioactive, its atoms will spontaneously 'decay', or turn into some other element, every so often. This means that the carbon14 atoms in a substance are constantly disappearing. But as long as the plant is alive, it will continue to take in both carbon and carbon14 atoms, so the ratio of the two types in the plant remains constant. Once the plant dies, however, the two types of carbon are no longer being replaced. Individual carbon14 atoms continue to decay, and disappear. Carbon14 decays (goes away) at a set, measurable rate. Once you know the rate, you can measure the Carbon14 that's left and get the date that the life-form died. That's how the Carbon14 dating method works. The decay of Carbon14 atoms is measured in multiples of 5,568 years. That's the "half-life." Half-life means that half of the Carbon14 will go away in 5,568 years. Of the remainder, half of that will go away in another 5,568 years. Generally by the time something is more then 40,000 years old, the Carbon14 is either too small to measure or has gone away completely. By measuring the reduced ratio of carbon14 atoms to regular carbon atoms in an artifact that was once alive, you can determine how long the carbon14 has been decaying, or how long it was since the living substance died, as long as that period is less than 40,000 years. For example, a piece of wood used in the construction of a spear could be dated. Carbon14 dating is particularly useful to archaeologists for establishing the age of objects found in the remains of ancient human settlements. The most famous example of the use of carbon14 dating was the testing of the Shroud of Turin. While claimed to be the burial cloth of Christ, carbon14 testing has dated it to the period 1260-1390 AD. That's the same time it first appeared, suggesting that it was a forgery. Because later contamination could affect the results, carbon14 dating is only approximately correct. We could, as an example, date the spear as being 4000 years old, give or take a few hundred years. Also, because most of the carbon14 will have disappeared after about 40,000 years, this dating method isn't useful for objects older than that. But it does give a reliable way to date most human artifacts back that far. Similar methods using longer-lived isotopes of other elements are used to date artifacts older than this. Find a more quantitative explanation of Carbon 14 dating here Resources