But it is possible to measure the time taken for half of the nuclei in a radioactive material to decay.
Half life can be defined as the time taken for the number of nuclei in a radioactive material to halve.
It can also be defined as the time taken for the count rate of a sample of radioactive material to fall to half of its starting level.
The first lesson, Isotopes of Pennies, introduces the idea of isotopes.
The final lesson, Frosty the Snowman Meets His Demise: An Analogy to Carbon Dating, is based on gathering evidence in the present and extrapolating it to the past.
"Established decontamination procedures were followed, which involves removing the contaminated clothing.