The starting isotope is called the parent and the end-product is called the daughter.
The time it takes for one half of the parent atoms to decay to the daughter atoms is called the half-life.
Some of the common isotope pairs used are K-Ar, Rb-Sr, Pb-Pb, and U-Pb.
Carbon-14 dating is another common technique, but it can only be used on carbon-containing things that were once alive.
If any of these three conditions is not accurately known, the hourglass will give an inaccurate measure of time.
Radiometric dating is based on the fact that radioactive isotopes decay to form isotopes of different elements.
Uniformitarian geologists use so-called dating methods to determine the ages of the surrounding rocks.