Explain what the term half-life means and how knowing the half-life of an element can help a geologist establish the age of a rock or fossil.?

Half-life refers to the time required for half of the radioactive atoms in a sample to decay into stable daughter isotopes. By measuring the ratio of parent isotopes to daughter products in a rock or fossil and knowing the half-life of the parent isotope, geologists can calculate the age of the sample. This technique, known as radiometric dating, provides a reliable means to determine the age of geological formations and fossils, allowing scientists to construct a timeline of Earth's history.