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.
In exponential decay the amount of material decreases by half during each half-life.
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.
It has become increasingly clear that these radiometric dating techniques agree with each other and as a whole, present a coherent picture in which the Earth was created a very long time ago.
Further evidence comes from the complete agreement between radiometric dates and other dating methods such as counting tree rings or glacier ice core layers.