A tracer can also be a label (usually a radioisotope, but sometimes a fluorescent molecule) added to a molecule to follow its progress through a (usually biological) pathway. For example, one can administer mescalin which has been labeled with carbon-11 to a rat and measure the distribution of mescalin and its metabolic products in the various organs of the rat with a Geiger counter (it is not known if the rats enjoy their trip.) Other commonly used tracers include deuterium, fluorine-18, oxygen-15 and green-fluorescent protein.