Carbon 14

You asked for an easy explanation

The original question asked for an easy way to explain it, so I'm going to try. Here goes . . . .

All organic matter contains carbon, which is an element. But there are different types of carbon, called isotopes.

The most common isotope is carbon-12 (or 12C), which (according the article) makes up 98.89 percent of the naturally occurring carbon. There's carbon-13, or 13C, which is much rarer, accounting for only 1.11 percent, and then there's carbon-14, or 14C, which makes up a ridiculously tiny fraction of existing carbon. (The periodic table of the elements also reflects the existence of isotopes by showing a weighted average for the atomic weight of each element, but I digress.)

The first two isotopes, 12C and 13C, are stable, but 14C is unstable; that is, it's radioactive!

So far, so good. Nothing hard to get your brain around.

Living organic matter will have steady and predictable concentrations of each isotope of carbon, pretty much the percentages mentioned above. But dead stuff won't. After something dies, the 14C 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). In other words, the amount of 14C in dead organic matter will grow smaller. And since scientists know exactly how long an amount of 14C takes to decay, they can compare the amount of 14C in a specimen to the amount of 14C a modern piece of organic matter and calculate the age of the specimen. Since it takes 5,568 years for an amount of 14C to decay by 50 percent (half), if a specimen has one half the amount of 14C 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. And say that every 5,568 years you add another gallon of water to the mixture. Doing that basically cuts the concentration of blue dye in half. You then take a gallon of that diluted mixture and add another gallon of pure water to it 5,568 years later. The concentration of blue dye is cut in half again. Now imagine repeating this process for quite some time. If you take a sample of the diluted water and measure the concentration of blue dye, you will be able to determine how many dilutions took place, and since you know the dilutions happen every 5,568 years, you can estimate how old the sample is.

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Answer
Carbon-14 builds up in living tissue at a constant rate and starts to break down when the tissue dies. Scientists can measure the amount of carbon-14 in a piece of old wood for instance, and say that because there is only a certain amount left, the tree died 1000 years ago.