That is a good question!  When we are faced with taking a drug test we often hear the word metabolite used.  Your metabolism is how your body processes the things that you ingest.  A metabolite is a byproduct of your metabolism.  When referring to a metabolite it is often used in the context of your body breaking down some type of medication or drug.  Drug metabolism usually occurs in the liver.  The liver contains enzymes which chemically change the components of the drugs into metabolites.  The metabolites are then linked to other substances for dispersal through the lungs or through bodily fluids like saliva, urine, sweat or even breast milk.  Some are even reabsorbed by your intestines.  Most metabolites travel through the kidneys before they are expelled from the body.  Metabolites from marijuana usage will stay in your system longer than other drugs because marijuana is a natural substance and your body doesn’t reject it like it would a man-made substance.  The marijuana metabolites are stored in your fat cells long after the effects of the marijuana wear off.  The residue from marijuana smoke is sticky and adheres to the lungs when you inhale the marijuana smoke so the resin is released into the bloodstream at a slower pace than substances that you swallow.  Each and every person has a different metabolism so the rates at which people metabolize drugs vary from person to person.  Genetics, age, nutrition and environment are also factors that play into how fast each individual metabolizes different substances.