Nephron is the basic structural and functional unit of the kidney. Its chief function is to regulate the concentration of water and soluble substances like sodium salts by filtering the blood, reabsorbing what is needed and excreting the rest as urine.
In an experiment, you are only allowed to test for one variable at a time. This means that everything else must be the same or constant for only the one variable to be tested for. If say you were testing if plants grow better with more or less water, you need to keep everything else the same.