The carbon-14 isotope is about average in life span. It has been determined that in any group of C-14 isotopes half of them will lose two of their ping-pong balls and become C-12 isotopes in 5,730 years, plus or minus 40. This is known as the _half-life_ of the C-14 isotope. The “plus or minus 40” allows for laboratory error in terms of years. Now all living things contain carbon atoms, and some of those atoms are C-14 isotopes. The amount of C-14 isotopes in a living organism quickly reaches a stable percentage after which there is no increase or decrease while the organism is alive. After it dies, the C-14 supply is not replenished, and with the passing of 5,730 plus or minus 40 years, it has half the number of C-14 isotopes it had when alive. In 11,460 plus or minus 80 years, it will have one quarter as many as it had when alive. The geochronologist takes a certain weight of carbon-bearing matter from an organism which once lived. With simple chemistry he can determine the number of carbon atoms in the material, usually charcoal, wood, bone or shell. He places the material in a chamber equipped with geiger counters and records the number of C-14 isotopes converting to C-12 isotopes within a certain number of hours or days. Since he knows how many carbon atoms there are in the specimen, he knows how many C-14 isotopes there would be if the specimen were alive. He also knows that as the number of C-14 atoms decreases the number of clicks on the geiger counter will decrease too. For example, if there are 2,000 C-14 isotopes, the decomposition of half of these over a period of approximately 5,730 years would register 1,000 clicks on the counter. In the next 5,730 year period there would be 1,000 isotopes left, and only 500 of them would decompose to register as geiger counter clicks. The geochronologist does the counting and analysis of the results and sends the information back to the archaeologist in the form of the number of years that have elapsed since the carbon was part of a living creature; for example, 1,500 plus or minus 150 years BP (before present). The archaeologist, converting this to the Christian calendar in 1964, would come up with A.D. 464 plus or minus 150 years. When was the sample actually alive? We don’t know exactly, but statistically we have a ninety-five percent chance of being right if we say sometime between A.D. 164 and 764. This clock ticks in centuries. But the radiocarbon clock doesn’t tick very long, even in centuries, before running down. By the time 30,000 to 40,000 years have gone by, the C-14 in any sample is almost gone, and there is too little left to give enough geiger counter clicks unless one is willing to wait a lifetime to record two or three clicks.