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You cannot predict exactly when any one particular grain will get to the bottom, but you can predict from one time to the next how long the whole pile of sand takes to fall.
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.
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.