When the glass is turned over, sand runs from the top to the bottom.
Radioactive atoms are like individual grains of sand--radioactive decays are like the falling of grains from the top to the bottom of the glass.
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.
Many Christians accept this and interpret the Genesis account in less scientifically literal ways.
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.
Most of the elements in nature are stable and do not change.
However, some elements are not completely stable in their natural state.
Many people have been led to be skeptical of dating without knowing much about it. In spite of this, differences still occur within the church.