No one can predict the moment when a particular grain will fall through the neck, but an estimate can be made for how long the whole pile of sand will take to fall.
Though most atoms contain stable nuclei and do not decay, some types do.
When radioactive decay occurs, no one can predict which individual atoms will decay when.
But, for a large number of atoms, the number that will decay within a given time can be predicted.
The original (parent) atom changes into a daughter atom having different chemical properties.
(For a brief science review, see sidebar and figure 1.) The timepiece that allows dating is the “radioactive” decay of certain kinds of atoms from one form into another.