Some of the atoms eventually change from one element to another by a process called radioactive decay.
If there are a lot of atoms of the original element, called the parent element, the atoms decay to another element, called the daughter element, at a predictable rate.
This paper describes in relatively simple terms how a number of the dating techniques work, how accurately the half-lives of the radioactive elements and the rock dates themselves are known, and how dates are checked with one another.
In the process the paper refutes a number of misconceptions prevalent among Christians today.
Even though the Earth's age is never mentioned in the Bible, it is an issue because those who take a strictly literal view of the early chapters of Genesis can calculate an approximate date for the creation by adding up the life-spans of the people mentioned in the genealogies.
Assuming a strictly literal interpretation of the week of creation, even if some of the generations were left out of the genealogies, the Earth would be less than ten thousand years old.
Most of the elements in nature are stable and do not change.
However, some elements are not completely stable in their natural state.
Further evidence comes from the complete agreement between radiometric dates and other dating methods such as counting tree rings or glacier ice core layers.
Similarly, when all the atoms of the radioactive element are gone, the rock will no longer keep time (unless it receives a new batch of radioactive atoms).
The rate of loss of sand from from the top of an hourglass compared to exponential type of decay of radioactive elements.
The passage of time can be charted by the reduction in the number of parent atoms, and the increase in the number of daughter atoms.
Radiometric dating can be compared to an hourglass.