Determining the age of the earth: 

A Rising Tide Project for Grades 9 and 10

Carbon 14 - A radioactive isotope found in all living organisms. The ratio of C14 to C12 in an organism is the same as the ratio in the surrounding atmosphere.  When the organism dies, the C14 in its body decays to N14 and the approximate age can be computed by comparing the percentages of C14 to C12.  This method is only good for ages up to about 60,000 years.

Half-Life – The time it takes for half of a mass of a radioactive element to decay into its daughter elements.