For example, about 1.5 percent of a quantity of Uranium 238 will decay to lead every 100 million years.
By measuring the ratio of lead to uranium in a rock sample, its age can be determined.
Using this technique, called radiometric dating, scientists are able to "see" back in time.
These rates are usually expressed as the isotope's half-life--that is, the time it takes for one-half of the parent isotopes to decay.
After one half-life, 50 percent of the original parents remains; after two, only 25 percent remains, and so on.
This means that as radioactive parent elements decay, they and their daughters are trapped together inside the crystal.
If, however, the rock is subjected to intense heat or pressure, some of the parent or daughter isotopes may be driven off.
Certain isotopes are unstable and undergo a process of radioactive decay, slowly and steadily transforming, molecule by molecule, into a different isotope.
This rate of decay is constant for a given isotope, and the time it takes for one-half of a particular isotope to decay is its radioactive half-life.
The age of the planet, though, was important to Charles Darwin and other evolutionary theorists: The biological evidence they were collecting showed that nature needed vastly more time than previously thought to sculpt the world.
A breakthrough came with the discovery of radioactivity at the beginning of the 1900s.
Some radioactive parent isotopes decay almost instantaneously into their stable daughter isotopes; others take billions of years.
The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures.
Many elements have naturally occurring isotopes, varieties of the element that have different numbers of neutrons in the nucleus.