Medical researchers use radioactivity in two general and important ways. First of all, they require radioactive isotopes as tags or tracers to study complex bodily processes such as the excretion of copper from the human system or the preferential uptake of isotopes by tumors. Secondly, researchers may induce radioactivity in a given biological sample in order to perform a quantitative analysis on it. This technique of activation analysis enables them to measure, for example, the amount of lead in hair and bone or to perform a multielemental analysis of blood.