When a person smokes cigarettes, precarcinogens in the form of tobacco smoke enter a person's lungs and lodge in the fat-soluble area of the cells. They become bound to proteins that convert the precarcinogens into carcinogens. These reactive molecules are quickly handed over to detoxifying proteins that make the carcinogens water-soluble and allow the body to eliminate them into the urine, before they can damage the cell.