A closed system with suitable known initial conditions lets isotope measurements estimate elapsed time. 100%, 50%, 25% and 12.5% represent zero, one, two and three half-lives. Fossils may be dated through associated suitable rocks rather than directly.

The model has a domain of validity. Explain its assumptions before using it in a changed context, then compare your prediction with evidence.

• A half-life is the time for half the parent radioactive atoms to decay. Repeated halvings connect remaining fraction to age.
• Half of the remainder decays each interval, not the same fixed fraction of the starting amount.