During the lifetime, organisms take this Carbon 14 from the atmosphere. Thus, maintaining a certain composition of Carbon 14 and Carbon 12 in them.They can use it to know the age of organic substances.When an organism dies, it stops consuming Carbon 14. Then, the amount of Carbon 14 in that organism decreases because it decays to Carbon 12. By knowing, the ratio of the amount of Carbon 14 and Carbon 12 of dead organisms, we would be able to estimate the organisms age. It is a standard tool used by archeologists.